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PERLRE(1)	       Perl Programmers	Reference Guide		     PERLRE(1)

NAME
       perlre -	Perl regular expressions

DESCRIPTION
       This page describes the syntax of regular expressions in	Perl.

       If you haven't used regular expressions before, a tutorial introduction
       is available in perlretut.  If you know just a little about them, a
       quick-start introduction	is available in	perlrequick.

       Except for "The Basics" section,	this page assumes you are familiar
       with regular expression basics, like what is a "pattern", what does it
       look like, and how it is	basically used.	 For a reference on how	they
       are used, plus various examples of the same, see	discussions of "m//",
       "s///", "qr//" and "??" in "Regexp Quote-Like Operators"	in perlop.

       New in v5.22, "use re 'strict'" applies stricter	rules than otherwise
       when compiling regular expression patterns.  It can find	things that,
       while legal, may	not be what you	intended.

   The Basics
       Regular expressions are strings with the	very particular	syntax and
       meaning described in this document and auxiliary	documents referred to
       by this one.  The strings are called "patterns".	 Patterns are used to
       determine if some other string, called the "target", has	(or doesn't
       have) the characteristics specified by the pattern.  We call this
       "matching" the target string against the	pattern.  Usually the match is
       done by having the target be the	first operand, and the pattern be the
       second operand, of one of the two binary	operators "=~" and "!~",
       listed in "Binding Operators" in	perlop;	and the	pattern	will have been
       converted from an ordinary string by one	of the operators in "Regexp
       Quote-Like Operators" in	perlop,	like so:

	$foo =~	m/abc/

       This evaluates to true if and only if the string	in the variable	$foo
       contains	somewhere in it, the sequence of characters "a", "b", then
       "c".  (The "=~ m", or match operator, is	described in
       "m/PATTERN/msixpodualngc" in perlop.)

       Patterns	that aren't already stored in some variable must be
       delimitted, at both ends, by delimitter characters.  These are often,
       as in the example above,	forward	slashes, and the typical way a pattern
       is written in documentation is with those slashes.  In most cases, the
       delimitter is the same character, fore and aft, but there are a few
       cases where a character looks like it has a mirror-image	mate, where
       the opening version is the beginning delimiter, and the closing one is
       the ending delimiter, like

	$foo =~	m<abc>

       Most times, the pattern is evaluated in double-quotish context, but it
       is possible to choose delimiters	to force single-quotish, like

	$foo =~	m'abc'

       If the pattern contains its delimiter within it,	that delimiter must be
       escaped.	 Prefixing it with a backslash (e.g., "/foo\/bar/") serves
       this purpose.

       Any single character in a pattern matches that same character in	the
       target string, unless the character is a	metacharacter with a special
       meaning described in this document.  A sequence of non-metacharacters
       matches the same	sequence in the	target string, as we saw above with
       "m/abc/".

       Only a few characters (all of them being	ASCII punctuation characters)
       are metacharacters.  The	most commonly used one is a dot	".", which
       normally	matches	almost any character (including	a dot itself).

       You can cause characters	that normally function as metacharacters to be
       interpreted literally by	prefixing them with a "\", just	like the
       pattern's delimiter must	be escaped if it also occurs within the
       pattern.	 Thus, "\." matches just a literal dot,	"." instead of its
       normal meaning.	This means that	the backslash is also a	metacharacter,
       so "\\" matches a single	"\".  And a sequence that contains an escaped
       metacharacter matches the same sequence (but without the	escape)	in the
       target string.  So, the pattern "/blur\\fl/" would match	any target
       string that contains the	sequence "blur\fl".

       The metacharacter "|" is	used to	match one thing	or another.  Thus

	$foo =~	m/this|that/

       is TRUE if and only if $foo contains either the sequence	"this" or the
       sequence	"that".	 Like all metacharacters, prefixing the	"|" with a
       backslash makes it match	the plain punctuation character; in its	case,
       the VERTICAL LINE.

	$foo =~	m/this\|that/

       is TRUE if and only if $foo contains the	sequence "this|that".

       You aren't limited to just a single "|".

	$foo =~	m/fee|fie|foe|fum/

       is TRUE if and only if $foo contains any	of those 4 sequences from the
       children's story	"Jack and the Beanstalk".

       As you can see, the "|" binds less tightly than a sequence of ordinary
       characters.  We can override this by using the grouping metacharacters,
       the parentheses "(" and ")".

	$foo =~	m/th(is|at) thing/

       is TRUE if and only if $foo contains either the sequence	"this thing"
       or the sequence "that thing".  The portions of the string that match
       the portions of the pattern enclosed in parentheses are normally	made
       available separately for	use later in the pattern, substitution,	or
       program.	 This is called	"capturing", and it can	get complicated.  See
       "Capture	groups".

       The first alternative includes everything from the last pattern
       delimiter ("(", "(?:" (described	later),	etc. or	the beginning of the
       pattern)	up to the first	"|", and the last alternative contains
       everything from the last	"|" to the next	closing	pattern	delimiter.
       That's why it's common practice to include alternatives in parentheses:
       to minimize confusion about where they start and	end.

       Alternatives are	tried from left	to right, so the first alternative
       found for which the entire expression matches, is the one that is
       chosen. This means that alternatives are	not necessarily	greedy.	For
       example:	when matching "foo|foot" against "barefoot", only the "foo"
       part will match,	as that	is the first alternative tried,	and it
       successfully matches the	target string. (This might not seem important,
       but it is important when	you are	capturing matched text using
       parentheses.)

       Besides taking away the special meaning of a metacharacter, a prefixed
       backslash changes some letter and digit characters away from matching
       just themselves to instead have special meaning.	 These are called
       "escape sequences", and all such	are described in perlrebackslash.  A
       backslash sequence (of a	letter or digit) that doesn't currently	have
       special meaning to Perl will raise a warning if warnings	are enabled,
       as those	are reserved for potential future use.

       One such	sequence is "\b", which	matches	a boundary of some sort.
       "\b{wb}"	and a few others give specialized types	of boundaries.	(They
       are all described in detail starting at "\b{}, \b, \B{},	\B" in
       perlrebackslash.)  Note that these don't	match characters, but the
       zero-width spaces between characters.  They are an example of a zero-
       width assertion.	 Consider again,

	$foo =~	m/fee|fie|foe|fum/

       It evaluates to TRUE if,	besides	those 4	words, any of the sequences
       "feed", "field",	"Defoe", "fume", and many others are in	$foo.  By
       judicious use of	"\b" (or better	(because it is designed	to handle
       natural language) "\b{wb}"), we can make	sure that only the Giant's
       words are matched:

	$foo =~	m/\b(fee|fie|foe|fum)\b/
	$foo =~	m/\b{wb}(fee|fie|foe|fum)\b{wb}/

       The final example shows that the	characters "{" and "}" are
       metacharacters.

       Another use for escape sequences	is to specify characters that cannot
       (or which you prefer not	to) be written literally.  These are described
       in detail in "Character Escapes"	in perlrebackslash, but	the next three
       paragraphs briefly describe some	of them.

       Various control characters can be written in C language style: "\n"
       matches a newline, "\t" a tab, "\r" a carriage return, "\f" a form
       feed, etc.

       More generally, "\nnn", where nnn is a string of	three octal digits,
       matches the character whose native code point is	nnn.  You can easily
       run into	trouble	if you don't have exactly three	digits.	 So always use
       three, or since Perl 5.14, you can use "\o{...}"	to specify any number
       of octal	digits.

       Similarly, "\xnn", where	nn are hexadecimal digits, matches the
       character whose native ordinal is nn.  Again, not using exactly two
       digits is a recipe for disaster,	but you	can use	"\x{...}" to specify
       any number of hex digits.

       Besides being a metacharacter, the "." is an example of a "character
       class", something that can match	any single character of	a given	set of
       them.  In its case, the set is just about all possible characters.
       Perl predefines several character classes besides the "."; there	is a
       separate	reference page about just these, perlrecharclass.

       You can define your own custom character	classes, by putting into your
       pattern in the appropriate place(s), a list of all the characters you
       want in the set.	 You do	this by	enclosing the list within "[]" bracket
       characters.  These are called "bracketed	character classes" when	we are
       being precise, but often	the word "bracketed" is	dropped.  (Dropping it
       usually doesn't cause confusion.)  This means that the "[" character is
       another metacharacter.  It doesn't match	anything just by itelf;	it is
       used only to tell Perl that what	follows	it is a	bracketed character
       class.  If you want to match a literal left square bracket, you must
       escape it, like "\[".  The matching "]" is also a metacharacter;	again
       it doesn't match	anything by itself, but	just marks the end of your
       custom class to Perl.  It is an example of a "sometimes metacharacter".
       It isn't	a metacharacter	if there is no corresponding "[", and matches
       its literal self:

	print "]" =~ /]/;  # prints 1

       The list	of characters within the character class gives the set of
       characters matched by the class.	 "[abc]" matches a single "a" or "b"
       or "c".	But if the first character after the "[" is "^", the class
       matches any character not in the	list.  Within a	list, the "-"
       character specifies a range of characters, so that "a-z"	represents all
       characters between "a" and "z", inclusive.  If you want either "-" or
       "]" itself to be	a member of a class, put it at the start of the	list
       (possibly after a "^"), or escape it with a backslash.  "-" is also
       taken literally when it is at the end of	the list, just before the
       closing "]".  (The following all	specify	the same class of three
       characters: "[-az]", "[az-]", and "[a\-z]".  All	are different from
       "[a-z]",	which specifies	a class	containing twenty-six characters, even
       on EBCDIC-based character sets.)

       There is	lots more to bracketed character classes; full details are in
       "Bracketed Character Classes" in	perlrecharclass.

       Metacharacters

       "The Basics" introduced some of the metacharacters.  This section gives
       them all.  Most of them have the	same meaning as	in the egrep command.

       Only the	"\" is always a	metacharacter.	The others are metacharacters
       just sometimes.	The following tables lists all of them,	summarizes
       their use, and gives the	contexts where they are	metacharacters.
       Outside those contexts or if prefixed by	a "\", they match their
       corresponding punctuation character.  In	some cases, their meaning
       varies depending	on various pattern modifiers that alter	the default
       behaviors.  See "Modifiers".

		   PURPOSE				    WHERE
	\   Escape the next character			 Always, except	when
							 escaped by another \
	^   Match the beginning	of the string		 Not in	[]
	      (or line,	if /m is used)
	^   Complement the [] class			 At the	beginning of []
	.   Match any single character except newline	 Not in	[]
	      (under /s, includes newline)
	$   Match the end of the string			 Not in	[], but	can
	      (or before newline at the	end of the	 mean interpolate a
	      string; or before	any newline if /m is	 scalar
	      used)
	|   Alternation					 Not in	[]
	()  Grouping					 Not in	[]
	[   Start Bracketed Character class		 Not in	[]
	]   End	Bracketed Character class		 Only in [], and
							   not first
	*   Matches the	preceding element 0 or more	 Not in	[]
	      times
	+   Matches the	preceding element 1 or more	 Not in	[]
	      times
	?   Matches the	preceding element 0 or 1	 Not in	[]
	      times
	{   Starts a sequence that gives number(s)	 Not in	[]
	      of times the preceding element can be
	      matched
	{   when following certain escape sequences
	      starts a modifier	to the meaning of the
	      sequence
	}   End	sequence started by {
	-   Indicates a	range				 Only in [] interior

       Notice that most	of the metacharacters lose their special meaning when
       they occur in a bracketed character class, except "^" has a different
       meaning when it is at the beginning of such a class.  And "-" and "]"
       are metacharacters only at restricted positions within bracketed
       character classes; while	"}" is a metacharacter only when closing a
       special construct started by "{".

       In double-quotish context, as is	usually	the case,  you need to be
       careful about "$" and the non-metacharacter "@".	 Those could
       interpolate variables, which may	or may not be what you intended.

       These rules were	designed for compactness of expression,	rather than
       legibility and maintainability.	The "/x	and /xx" pattern modifiers
       allow you to insert white space to improve readability.	And use	of
       "re 'strict'" adds extra	checking to catch some typos that might
       silently	compile	into something unintended.

       By default, the "^" character is	guaranteed to match only the beginning
       of the string, the "$" character	only the end (or before	the newline at
       the end), and Perl does certain optimizations with the assumption that
       the string contains only	one line.  Embedded newlines will not be
       matched by "^" or "$".  You may,	however, wish to treat a string	as a
       multi-line buffer, such that the	"^" will match after any newline
       within the string (except if the	newline	is the last character in the
       string),	and "$"	will match before any newline.	At the cost of a
       little more overhead, you can do	this by	using the ""/m"" modifier on
       the pattern match operator.  (Older programs did	this by	setting	$*,
       but this	option was removed in perl 5.10.)

       To simplify multi-line substitutions, the "." character never matches a
       newline unless you use the "/s" modifier, which in effect tells Perl to
       pretend the string is a single line--even if it isn't.

   Modifiers
       Overview

       The default behavior for	matching can be	changed, using various
       modifiers.  Modifiers that relate to the	interpretation of the pattern
       are listed just below.  Modifiers that alter the	way a pattern is used
       by Perl are detailed in "Regexp Quote-Like Operators" in	perlop and
       "Gory details of	parsing	quoted constructs" in perlop.

       "m" Treat the string being matched against as multiple lines.  That is,
	   change "^" and "$" from matching the	start of the string's first
	   line	and the	end of its last	line to	matching the start and end of
	   each	line within the	string.

       "s" Treat the string as single line.  That is, change "." to match any
	   character whatsoever, even a	newline, which normally	it would not
	   match.

	   Used	together, as "/ms", they let the "." match any character
	   whatsoever, while still allowing "^"	and "$"	to match,
	   respectively, just after and	just before newlines within the
	   string.

       "i" Do case-insensitive pattern matching.  For example, "A" will	match
	   "a" under "/i".

	   If locale matching rules are	in effect, the case map	is taken from
	   the current locale for code points less than	255, and from Unicode
	   rules for larger code points.  However, matches that	would cross
	   the Unicode rules/non-Unicode rules boundary	(ords 255/256) will
	   not succeed,	unless the locale is a UTF-8 one.  See perllocale.

	   There are a number of Unicode characters that match a sequence of
	   multiple characters under "/i".  For	example, "LATIN	SMALL LIGATURE
	   FI" should match the	sequence "fi".	Perl is	not currently able to
	   do this when	the multiple characters	are in the pattern and are
	   split between groupings, or when one	or more	are quantified.	 Thus

	    "\N{LATIN SMALL LIGATURE FI}" =~ /fi/i;	     # Matches
	    "\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i;    # Doesn't match!
	    "\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i;	     # Doesn't match!

	    # The below	doesn't	match, and it isn't clear what $1 and $2 would
	    # be even if it did!!
	    "\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i;	     # Doesn't match!

	   Perl	doesn't	match multiple characters in a bracketed character
	   class unless	the character that maps	to them	is explicitly
	   mentioned, and it doesn't match them	at all if the character	class
	   is inverted,	which otherwise	could be highly	confusing.  See
	   "Bracketed Character	Classes" in perlrecharclass, and "Negation" in
	   perlrecharclass.

       "x" and "xx"
	   Extend your pattern's legibility by permitting whitespace and
	   comments.  Details in "/x and  /xx"

       "p" Preserve the	string matched such that "${^PREMATCH}", "${^MATCH}",
	   and "${^POSTMATCH}" are available for use after matching.

	   In Perl 5.20	and higher this	is ignored. Due	to a new copy-on-write
	   mechanism, "${^PREMATCH}", "${^MATCH}", and "${^POSTMATCH}" will be
	   available after the match regardless	of the modifier.

       "a", "d", "l", and "u"
	   These modifiers, all	new in 5.14, affect which character-set	rules
	   (Unicode, etc.) are used, as	described below	in "Character set
	   modifiers".

       "n" Prevent the grouping	metacharacters "()" from capturing. This
	   modifier, new in 5.22, will stop $1,	$2, etc... from	being filled
	   in.

	     "hello" =~	/(hi|hello)/;	# $1 is	"hello"
	     "hello" =~	/(hi|hello)/n;	# $1 is	undef

	   This	is equivalent to putting "?:" at the beginning of every
	   capturing group:

	     "hello" =~	/(?:hi|hello)/;	# $1 is	undef

	   "/n"	can be negated on a per-group basis. Alternatively, named
	   captures may	still be used.

	     "hello" =~	/(?-n:(hi|hello))/n;   # $1 is "hello"
	     "hello" =~	/(?<greet>hi|hello)/n; # $1 is "hello",	$+{greet} is
					       # "hello"

       Other Modifiers
	   There are a number of flags that can	be found at the	end of regular
	   expression constructs that are not generic regular expression
	   flags, but apply to the operation being performed, like matching or
	   substitution	("m//" or "s///" respectively).

	   Flags described further in "Using regular expressions in Perl" in
	   perlretut are:

	     c	- keep the current position during repeated matching
	     g	- globally match the pattern repeatedly	in the string

	   Substitution-specific modifiers described in
	   "s/PATTERN/REPLACEMENT/msixpodualngcer" in perlop are:

	     e	- evaluate the right-hand side as an expression
	     ee	- evaluate the right side as a string then eval	the result
	     o	- pretend to optimize your code, but actually introduce	bugs
	     r	- perform non-destructive substitution and return the new value

       Regular expression modifiers are	usually	written	in documentation as
       e.g., "the "/x" modifier", even though the delimiter in question	might
       not really be a slash.  The modifiers "/imnsxadlup" may also be
       embedded	within the regular expression itself using the "(?...)"
       construct, see "Extended	Patterns" below.

       Details on some modifiers

       Some of the modifiers require more explanation than given in the
       "Overview" above.

       "/x" and	 "/xx"

       A single	"/x" tells the regular expression parser to ignore most
       whitespace that is neither backslashed nor within a bracketed character
       class.  You can use this	to break up your regular expression into more
       readable	parts.	Also, the "#" character	is treated as a	metacharacter
       introducing a comment that runs up to the pattern's closing delimiter,
       or to the end of	the current line if the	pattern	extends	onto the next
       line.  Hence, this is very much like an ordinary	Perl code comment.
       (You can	include	the closing delimiter within the comment only if you
       precede it with a backslash, so be careful!)

       Use of "/x" means that if you want real whitespace or "#" characters in
       the pattern (outside a bracketed	character class, which is unaffected
       by "/x"), then you'll either have to escape them	(using backslashes or
       "\Q...\E") or encode them using octal, hex, or "\N{}" escapes.  It is
       ineffective to try to continue a	comment	onto the next line by escaping
       the "\n"	with a backslash or "\Q".

       You can use "(?#text)" to create	a comment that ends earlier than the
       end of the current line,	but "text" also	can't contain the closing
       delimiter unless	escaped	with a backslash.

       A common	pitfall	is to forget that "#" characters begin a comment under
       "/x" and	are not	matched	literally.  Just keep that in mind when	trying
       to puzzle out why a particular "/x" pattern isn't working as expected.

       Starting	in Perl	v5.26, if the modifier has a second "x"	within it, it
       does everything that a single "/x" does,	but additionally non-
       backslashed SPACE and TAB characters within bracketed character classes
       are also	generally ignored, and hence can be added to make the classes
       more readable.

	   / [d-e g-i 3-7]/xx
	   /[ !	@ " # $	% ^ & *	() = ? <> ' ]/xx

       may be easier to	grasp than the squashed	equivalents

	   /[d-eg-i3-7]/
	   /[!@"#$%^&*()=?<>']/

       Taken together, these features go a long	way towards making Perl's
       regular expressions more	readable.  Here's an example:

	   # Delete (most) C comments.
	   $program =~ s {
	       /\*     # Match the opening delimiter.
	       .*?     # Match a minimal number	of characters.
	       \*/     # Match the closing delimiter.
	   } []gsx;

       Note that anything inside a "\Q...\E" stays unaffected by "/x".	And
       note that "/x" doesn't affect space interpretation within a single
       multi-character construct.  For example in "\x{...}", regardless	of the
       "/x" modifier, there can	be no spaces.  Same for	a quantifier such as
       "{3}" or	"{5,}".	 Similarly, "(?:...)" can't have a space between the
       "{", "?", and ":".  Within any delimiters for such a construct, allowed
       spaces are not affected by "/x",	and depend on the construct.  For
       example,	"\x{...}" can't	have spaces because hexadecimal	numbers	don't
       have spaces in them.  But, Unicode properties can have spaces, so in
       "\p{...}" there can be spaces that follow the Unicode rules, for	which
       see "Properties accessible through \p{} and \P{}" in perluniprops.

       The set of characters that are deemed whitespace	are those that Unicode
       calls "Pattern White Space", namely:

	U+0009 CHARACTER TABULATION
	U+000A LINE FEED
	U+000B LINE TABULATION
	U+000C FORM FEED
	U+000D CARRIAGE	RETURN
	U+0020 SPACE
	U+0085 NEXT LINE
	U+200E LEFT-TO-RIGHT MARK
	U+200F RIGHT-TO-LEFT MARK
	U+2028 LINE SEPARATOR
	U+2029 PARAGRAPH SEPARATOR

       Character set modifiers

       "/d", "/u", "/a", and "/l", available starting in 5.14, are called the
       character set modifiers;	they affect the	character set rules used for
       the regular expression.

       The "/d", "/u", and "/l"	modifiers are not likely to be of much use to
       you, and	so you need not	worry about them very much.  They exist	for
       Perl's internal use, so that complex regular expression data structures
       can be automatically serialized and later exactly reconstituted,
       including all their nuances.  But, since	Perl can't keep	a secret, and
       there may be rare instances where they are useful, they are documented
       here.

       The "/a"	modifier, on the other hand, may be useful.  Its purpose is to
       allow code that is to work mostly on ASCII data to not have to concern
       itself with Unicode.

       Briefly,	"/l" sets the character	set to that of whatever	Locale is in
       effect at the time of the execution of the pattern match.

       "/u" sets the character set to Unicode.

       "/a" also sets the character set	to Unicode, BUT	adds several
       restrictions for	ASCII-safe matching.

       "/d" is the old,	problematic, pre-5.14 Default character	set behavior.
       Its only	use is to force	that old behavior.

       At any given time, exactly one of these modifiers is in effect.	Their
       existence allows	Perl to	keep the originally compiled behavior of a
       regular expression, regardless of what rules are	in effect when it is
       actually	executed.  And if it is	interpolated into a larger regex, the
       original's rules	continue to apply to it, and only it.

       The "/l"	and "/u" modifiers are automatically selected for regular
       expressions compiled within the scope of	various	pragmas, and we
       recommend that in general, you use those	pragmas	instead	of specifying
       these modifiers explicitly.  For	one thing, the modifiers affect	only
       pattern matching, and do	not extend to even any replacement done,
       whereas using the pragmas gives consistent results for all appropriate
       operations within their scopes.	For example,

	s/foo/\Ubar/il

       will match "foo"	using the locale's rules for case-insensitive
       matching, but the "/l" does not affect how the "\U" operates.  Most
       likely you want both of them to use locale rules.  To do	this, instead
       compile the regular expression within the scope of "use locale".	 This
       both implicitly adds the	"/l", and applies locale rules to the "\U".
       The lesson is to	"use locale", and not "/l" explicitly.

       Similarly, it would be better to	use "use feature 'unicode_strings'"
       instead of,

	s/foo/\Lbar/iu

       to get Unicode rules, as	the "\L" in the	former (but not	necessarily
       the latter) would also use Unicode rules.

       More detail on each of the modifiers follows.  Most likely you don't
       need to know this detail	for "/l", "/u",	and "/d", and can skip ahead
       to /a.

       /l

       means to	use the	current	locale's rules (see perllocale)	when pattern
       matching.  For example, "\w" will match the "word" characters of	that
       locale, and "/i"	case-insensitive matching will match according to the
       locale's	case folding rules.  The locale	used will be the one in	effect
       at the time of execution	of the pattern match.  This may	not be the
       same as the compilation-time locale, and	can differ from	one match to
       another if there	is an intervening call of the setlocale() function.

       Prior to	v5.20, Perl did	not support multi-byte locales.	 Starting
       then, UTF-8 locales are supported.  No other multi byte locales are
       ever likely to be supported.  However, in all locales, one can have
       code points above 255 and these will always be treated as Unicode no
       matter what locale is in	effect.

       Under Unicode rules, there are a	few case-insensitive matches that
       cross the 255/256 boundary.  Except for UTF-8 locales in	Perls v5.20
       and later, these	are disallowed under "/l".  For	example, 0xFF (on
       ASCII platforms)	does not caselessly match the character	at 0x178,
       "LATIN CAPITAL LETTER Y WITH DIAERESIS",	because	0xFF may not be	"LATIN
       SMALL LETTER Y WITH DIAERESIS" in the current locale, and Perl has no
       way of knowing if that character	even exists in the locale, much	less
       what code point it is.

       In a UTF-8 locale in v5.20 and later, the only visible difference
       between locale and non-locale in	regular	expressions should be tainting
       (see perlsec).

       This modifier may be specified to be the	default	by "use	locale", but
       see "Which character set	modifier is in effect?".

       /u

       means to	use Unicode rules when pattern matching.  On ASCII platforms,
       this means that the code	points between 128 and 255 take	on their
       Latin-1 (ISO-8859-1) meanings (which are	the same as Unicode's).
       (Otherwise Perl considers their meanings	to be undefined.)  Thus, under
       this modifier, the ASCII	platform effectively becomes a Unicode
       platform; and hence, for	example, "\w" will match any of	the more than
       100_000 word characters in Unicode.

       Unlike most locales, which are specific to a language and country pair,
       Unicode classifies all the characters that are letters somewhere	in the
       world as	"\w".  For example, your locale	might not think	that "LATIN
       SMALL LETTER ETH" is a letter (unless you happen	to speak Icelandic),
       but Unicode does.  Similarly, all the characters	that are decimal
       digits somewhere	in the world will match	"\d"; this is hundreds,	not
       10, possible matches.  And some of those	digits look like some of the
       10 ASCII	digits,	but mean a different number, so	a human	could easily
       think a number is a different quantity than it really is.  For example,
       "BENGALI	DIGIT FOUR" (U+09EA) looks very	much like an "ASCII DIGIT
       EIGHT" (U+0038).	 And, "\d+", may match strings of digits that are a
       mixture from different writing systems, creating	a security issue.
       "num()" in Unicode::UCD can be used to sort this	out.  Or the "/a"
       modifier	can be used to force "\d" to match just	the ASCII 0 through 9.

       Also, under this	modifier, case-insensitive matching works on the full
       set of Unicode characters.  The "KELVIN SIGN", for example matches the
       letters "k" and "K"; and	"LATIN SMALL LIGATURE FF" matches the sequence
       "ff", which, if you're not prepared, might make it look like a
       hexadecimal constant, presenting	another	potential security issue.  See
       <http://unicode.org/reports/tr36> for a detailed	discussion of Unicode
       security	issues.

       This modifier may be specified to be the	default	by "use	feature
       'unicode_strings", "use locale ':not_characters'", or "use 5.012" (or
       higher),	but see	"Which character set modifier is in effect?".

       /d

       This modifier means to use the "Default"	native rules of	the platform
       except when there is cause to use Unicode rules instead,	as follows:

       1.  the target string is	encoded	in UTF-8; or

       2.  the pattern is encoded in UTF-8; or

       3.  the pattern explicitly mentions a code point	that is	above 255 (say
	   by "\x{100}"); or

       4.  the pattern uses a Unicode name ("\N{...}");	 or

       5.  the pattern uses a Unicode property ("\p{...}" or "\P{...}"); or

       6.  the pattern uses a Unicode break ("\b{...}" or "\B{...}"); or

       7.  the pattern uses ""(?[ ])""

       Another mnemonic	for this modifier is "Depends",	as the rules actually
       used depend on various things, and as a result you can get unexpected
       results.	 See "The "Unicode Bug"" in perlunicode.  The Unicode Bug has
       become rather infamous, leading to yet another (printable) name for
       this modifier, "Dodgy".

       Unless the pattern or string are	encoded	in UTF-8, only ASCII
       characters can match positively.

       Here are	some examples of how that works	on an ASCII platform:

	$str =	"\xDF";	     # $str is not in UTF-8 format.
	$str =~	/^\w/;	     # No match, as $str isn't in UTF-8	format.
	$str .=	"\x{0e0b}";  # Now $str	is in UTF-8 format.
	$str =~	/^\w/;	     # Match! $str is now in UTF-8 format.
	chop $str;
	$str =~	/^\w/;	     # Still a match! $str remains in UTF-8 format.

       This modifier is	automatically selected by default when none of the
       others are, so yet another name for it is "Default".

       Because of the unexpected behaviors associated with this	modifier, you
       probably	should only explicitly use it to maintain weird	backward
       compatibilities.

       /a (and /aa)

       This modifier stands for	ASCII-restrict (or ASCII-safe).	 This modifier
       may be doubled-up to increase its effect.

       When it appears singly, it causes the sequences "\d", "\s", "\w", and
       the Posix character classes to match only in the	ASCII range.  They
       thus revert to their pre-5.6, pre-Unicode meanings.  Under "/a",	 "\d"
       always means precisely the digits "0" to	"9"; "\s" means	the five
       characters "[ \f\n\r\t]", and starting in Perl v5.18, the vertical tab;
       "\w" means the 63 characters "[A-Za-z0-9_]"; and	likewise, all the
       Posix classes such as "[[:print:]]" match only the appropriate ASCII-
       range characters.

       This modifier is	useful for people who only incidentally	use Unicode,
       and who do not wish to be burdened with its complexities	and security
       concerns.

       With "/a", one can write	"\d" with confidence that it will only match
       ASCII characters, and should the	need arise to match beyond ASCII, you
       can instead use "\p{Digit}" (or "\p{Word}" for "\w").  There are
       similar "\p{...}" constructs that can match beyond ASCII	both white
       space (see "Whitespace" in perlrecharclass), and	Posix classes (see
       "POSIX Character	Classes" in perlrecharclass).  Thus, this modifier
       doesn't mean you	can't use Unicode, it means that to get	Unicode
       matching	you must explicitly use	a construct ("\p{}", "\P{}") that
       signals Unicode.

       As you would expect, this modifier causes, for example, "\D" to mean
       the same	thing as "[^0-9]"; in fact, all	non-ASCII characters match
       "\D", "\S", and "\W".  "\b" still means to match	at the boundary
       between "\w" and	"\W", using the	"/a" definitions of them (similarly
       for "\B").

       Otherwise, "/a" behaves like the	"/u" modifier, in that case-
       insensitive matching uses Unicode rules;	for example, "k" will match
       the Unicode "\N{KELVIN SIGN}" under "/i"	matching, and code points in
       the Latin1 range, above ASCII will have Unicode rules when it comes to
       case-insensitive	matching.

       To forbid ASCII/non-ASCII matches (like "k" with	"\N{KELVIN SIGN}"),
       specify the "a" twice, for example "/aai" or "/aia".  (The first
       occurrence of "a" restricts the "\d", etc., and the second occurrence
       adds the	"/i" restrictions.)  But, note that code points	outside	the
       ASCII range will	use Unicode rules for "/i" matching, so	the modifier
       doesn't really restrict things to just ASCII; it	just forbids the
       intermixing of ASCII and	non-ASCII.

       To summarize, this modifier provides protection for applications	that
       don't wish to be	exposed	to all of Unicode.  Specifying it twice	gives
       added protection.

       This modifier may be specified to be the	default	by "use	re '/a'" or
       "use re '/aa'".	If you do so, you may actually have occasion to	use
       the "/u"	modifier explicitly if there are a few regular expressions
       where you do want full Unicode rules (but even here, it's best if
       everything were under feature "unicode_strings",	along with the "use re
       '/aa'").	 Also see "Which character set modifier	is in effect?".

       Which character set modifier is in effect?

       Which of	these modifiers	is in effect at	any given point	in a regular
       expression depends on a fairly complex set of interactions.  These have
       been designed so	that in	general	you don't have to worry	about it, but
       this section gives the gory details.  As	explained below	in "Extended
       Patterns" it is possible	to explicitly specify modifiers	that apply
       only to portions	of a regular expression.  The innermost	always has
       priority	over any outer ones, and one applying to the whole expression
       has priority over any of	the default settings that are described	in the
       remainder of this section.

       The "use	re '/foo'" pragma can be used to set default modifiers
       (including these) for regular expressions compiled within its scope.
       This pragma has precedence over the other pragmas listed	below that
       also change the defaults.

       Otherwise, "use locale" sets the	default	modifier to "/l"; and "use
       feature 'unicode_strings", or "use 5.012" (or higher) set the default
       to "/u" when not	in the same scope as either "use locale" or "use
       bytes".	("use locale ':not_characters'"	also sets the default to "/u",
       overriding any plain "use locale".)  Unlike the mechanisms mentioned
       above, these affect operations besides regular expressions pattern
       matching, and so	give more consistent results with other	operators,
       including using "\U", "\l", etc.	in substitution	replacements.

       If none of the above apply, for backwards compatibility reasons,	the
       "/d" modifier is	the one	in effect by default.  As this can lead	to
       unexpected results, it is best to specify which other rule set should
       be used.

       Character set modifier behavior prior to	Perl 5.14

       Prior to	5.14, there were no explicit modifiers,	but "/l" was implied
       for regexes compiled within the scope of	"use locale", and "/d" was
       implied otherwise.  However, interpolating a regex into a larger	regex
       would ignore the	original compilation in	favor of whatever was in
       effect at the time of the second	compilation.  There were a number of
       inconsistencies (bugs) with the "/d" modifier, where Unicode rules
       would be	used when inappropriate, and vice versa.  "\p{}" did not imply
       Unicode rules, and neither did all occurrences of "\N{}", until 5.12.

   Regular Expressions
       Quantifiers

       Quantifiers are used when a particular portion of a pattern needs to
       match a certain number (or numbers) of times.  If there isn't a
       quantifier the number of	times to match is exactly one.	The following
       standard	quantifiers are	recognized:

	   *	       Match 0 or more times
	   +	       Match 1 or more times
	   ?	       Match 1 or 0 times
	   {n}	       Match exactly n times
	   {n,}	       Match at	least n	times
	   {n,m}       Match at	least n	but not	more than m times

       (If a non-escaped curly bracket occurs in a context other than one of
       the quantifiers listed above, where it does not form part of a
       backslashed sequence like "\x{...}", it is either a fatal syntax	error,
       or treated as a regular character, generally with a deprecation warning
       raised.	To escape it, you can precede it with a	backslash ("\{") or
       enclose it within square	brackets  ("[{]").  This change	will allow for
       future syntax extensions	(like making the lower bound of	a quantifier
       optional), and better error checking of quantifiers).

       The "*" quantifier is equivalent	to "{0,}", the "+" quantifier to
       "{1,}", and the "?" quantifier to "{0,1}".  n and m are limited to non-
       negative	integral values	less than a preset limit defined when perl is
       built.  This is usually 32766 on	the most common	platforms.  The	actual
       limit can be seen in the	error message generated	by code	such as	this:

	   $_ **= $_ , / {$_} /	for 2 .. 42;

       By default, a quantified	subpattern is "greedy",	that is, it will match
       as many times as	possible (given	a particular starting location)	while
       still allowing the rest of the pattern to match.	 If you	want it	to
       match the minimum number	of times possible, follow the quantifier with
       a "?".  Note that the meanings don't change, just the "greediness":

	   *?	     Match 0 or	more times, not	greedily
	   +?	     Match 1 or	more times, not	greedily
	   ??	     Match 0 or	1 time,	not greedily
	   {n}?	     Match exactly n times, not	greedily (redundant)
	   {n,}?     Match at least n times, not greedily
	   {n,m}?    Match at least n but not more than	m times, not greedily

       Normally	when a quantified subpattern does not allow the	rest of	the
       overall pattern to match, Perl will backtrack. However, this behaviour
       is sometimes undesirable. Thus Perl provides the	"possessive"
       quantifier form as well.

	*+     Match 0 or more times and give nothing back
	++     Match 1 or more times and give nothing back
	?+     Match 0 or 1 time and give nothing back
	{n}+   Match exactly n times and give nothing back (redundant)
	{n,}+  Match at	least n	times and give nothing back
	{n,m}+ Match at	least n	but not	more than m times and give nothing back

       For instance,

	  'aaaa' =~ /a++a/

       will never match, as the	"a++" will gobble up all the "a"'s in the
       string and won't	leave any for the remaining part of the	pattern. This
       feature can be extremely	useful to give perl hints about	where it
       shouldn't backtrack. For	instance, the typical "match a double-quoted
       string" problem can be most efficiently performed when written as:

	  /"(?:[^"\\]++|\\.)*+"/

       as we know that if the final quote does not match, backtracking will
       not help. See the independent subexpression ""(?>pattern)"" for more
       details;	possessive quantifiers are just	syntactic sugar	for that
       construct. For instance the above example could also be written as
       follows:

	  /"(?>(?:(?>[^"\\]+)|\\.)*)"/

       Note that the possessive	quantifier modifier can	not be be combined
       with the	non-greedy modifier. This is because it	would make no sense.
       Consider	the follow equivalency table:

	   Illegal	   Legal
	   ------------	   ------
	   X??+		   X{0}
	   X+?+		   X{1}
	   X{min,max}?+	   X{min}

       Escape sequences

       Because patterns	are processed as double-quoted strings,	the following
       also work:

	\t	    tab			  (HT, TAB)
	\n	    newline		  (LF, NL)
	\r	    return		  (CR)
	\f	    form feed		  (FF)
	\a	    alarm (bell)	  (BEL)
	\e	    escape (think troff)  (ESC)
	\cK	    control char	  (example: VT)
	\x{}, \x00  character whose ordinal is the given hexadecimal number
	\N{name}    named Unicode character or character sequence
	\N{U+263D}  Unicode character	  (example: FIRST QUARTER MOON)
	\o{}, \000  character whose ordinal is the given octal number
	\l	    lowercase next char	(think vi)
	\u	    uppercase next char	(think vi)
	\L	    lowercase until \E (think vi)
	\U	    uppercase until \E (think vi)
	\Q	    quote (disable) pattern metacharacters until \E
	\E	    end	either case modification or quoted section, think vi

       Details are in "Quote and Quote-like Operators" in perlop.

       Character Classes and other Special Escapes

       In addition, Perl defines the following:

	Sequence   Note	   Description
	 [...]	   [1]	Match a	character according to the rules of the
			  bracketed character class defined by the "...".
			  Example: [a-z] matches "a" or	"b" or "c" ... or "z"
	 [[:...:]] [2]	Match a	character according to the rules of the	POSIX
			  character class "..."	within the outer bracketed
			  character class.  Example: [[:upper:]] matches any
			  uppercase character.
	 (?[...])  [8]	Extended bracketed character class
	 \w	   [3]	Match a	"word" character (alphanumeric plus "_", plus
			  other	connector punctuation chars plus Unicode
			  marks)
	 \W	   [3]	Match a	non-"word" character
	 \s	   [3]	Match a	whitespace character
	 \S	   [3]	Match a	non-whitespace character
	 \d	   [3]	Match a	decimal	digit character
	 \D	   [3]	Match a	non-digit character
	 \pP	   [3]	Match P, named property.  Use \p{Prop} for longer names
	 \PP	   [3]	Match non-P
	 \X	   [4]	Match Unicode "eXtended	grapheme cluster"
	 \1	   [5]	Backreference to a specific capture group or buffer.
			  '1' may actually be any positive integer.
	 \g1	   [5]	Backreference to a specific or previous	group,
	 \g{-1}	   [5]	The number may be negative indicating a	relative
			  previous group and may optionally be wrapped in
			  curly	brackets for safer parsing.
	 \g{name}  [5]	Named backreference
	 \k<name>  [5]	Named backreference
	 \K	   [6]	Keep the stuff left of the \K, don't include it	in $&
	 \N	   [7]	Any character but \n.  Not affected by /s modifier
	 \v	   [3]	Vertical whitespace
	 \V	   [3]	Not vertical whitespace
	 \h	   [3]	Horizontal whitespace
	 \H	   [3]	Not horizontal whitespace
	 \R	   [4]	Linebreak

       [1] See "Bracketed Character Classes" in	perlrecharclass	for details.

       [2] See "POSIX Character	Classes" in perlrecharclass for	details.

       [3] See "Backslash sequences" in	perlrecharclass	for details.

       [4] See "Misc" in perlrebackslash for details.

       [5] See "Capture	groups"	below for details.

       [6] See "Extended Patterns" below for details.

       [7] Note	that "\N" has two meanings.  When of the form "\N{NAME}", it
	   matches the character or character sequence whose name is "NAME";
	   and similarly when of the form "\N{U+hex}", it matches the
	   character whose Unicode code	point is hex.  Otherwise it matches
	   any character but "\n".

       [8] See "Extended Bracketed Character Classes" in perlrecharclass for
	   details.

       Assertions

       Besides "^" and "$", Perl defines the following zero-width assertions:

	\b{}   Match at	Unicode	boundary of specified type
	\B{}   Match where corresponding \b{} doesn't match
	\b     Match a \w\W or \W\w boundary
	\B     Match except at a \w\W or \W\w boundary
	\A     Match only at beginning of string
	\Z     Match only at end of string, or before newline at the end
	\z     Match only at end of string
	\G     Match only at pos() (e.g. at the	end-of-match position
	       of prior	m//g)

       A Unicode boundary ("\b{}"), available starting in v5.22, is a spot
       between two characters, or before the first character in	the string, or
       after the final character in the	string where certain criteria defined
       by Unicode are met.  See	"\b{}, \b, \B{}, \B" in	perlrebackslash	for
       details.

       A word boundary ("\b") is a spot	between	two characters that has	a "\w"
       on one side of it and a "\W" on the other side of it (in	either order),
       counting	the imaginary characters off the beginning and end of the
       string as matching a "\W".  (Within character classes "\b" represents
       backspace rather	than a word boundary, just as it normally does in any
       double-quoted string.)  The "\A"	and "\Z" are just like "^" and "$",
       except that they	won't match multiple times when	the "/m" modifier is
       used, while "^" and "$" will match at every internal line boundary.  To
       match the actual	end of the string and not ignore an optional trailing
       newline,	use "\z".

       The "\G"	assertion can be used to chain global matches (using "m//g"),
       as described in "Regexp Quote-Like Operators" in	perlop.	 It is also
       useful when writing "lex"-like scanners,	when you have several patterns
       that you	want to	match against consequent substrings of your string;
       see the previous	reference.  The	actual location	where "\G" will	match
       can also	be influenced by using "pos()" as an lvalue: see "pos" in
       perlfunc. Note that the rule for	zero-length matches (see "Repeated
       Patterns	Matching a Zero-length Substring") is modified somewhat, in
       that contents to	the left of "\G" are not counted when determining the
       length of the match. Thus the following will not	match forever:

	    my $string = 'ABC';
	    pos($string) = 1;
	    while ($string =~ /(.\G)/g)	{
		print $1;
	    }

       It will print 'A' and then terminate, as	it considers the match to be
       zero-width, and thus will not match at the same position	twice in a
       row.

       It is worth noting that "\G" improperly used can	result in an infinite
       loop. Take care when using patterns that	include	"\G" in	an
       alternation.

       Note also that "s///" will refuse to overwrite part of a	substitution
       that has	already	been replaced; so for example this will	stop after the
       first iteration,	rather than iterating its way backwards	through	the
       string:

	   $_ =	"123456789";
	   pos = 6;
	   s/.(?=.\G)/X/g;
	   print;      # prints	1234X6789, not XXXXX6789

       Capture groups

       The grouping construct "( ... )"	creates	capture	groups (also referred
       to as capture buffers). To refer	to the current contents	of a group
       later on, within	the same pattern, use "\g1" (or	"\g{1}") for the
       first, "\g2" (or	"\g{2}") for the second, and so	on.  This is called a
       backreference.

       There is	no limit to the	number of captured substrings that you may
       use.  Groups are	numbered with the leftmost open	parenthesis being
       number 1, etc.  If a group did not match, the associated	backreference
       won't match either. (This can happen if the group is optional, or in a
       different branch	of an alternation.)  You can omit the "g", and write
       "\1", etc, but there are	some issues with this form, described below.

       You can also refer to capture groups relatively,	by using a negative
       number, so that "\g-1" and "\g{-1}" both	refer to the immediately
       preceding capture group,	and "\g-2" and "\g{-2}"	both refer to the
       group before it.	 For example:

	       /
		(Y)	       # group 1
		(	       # group 2
		   (X)	       # group 3
		   \g{-1}      # backref to group 3
		   \g{-3}      # backref to group 1
		)
	       /x

       would match the same as "/(Y) ( (X) \g3 \g1 )/x".  This allows you to
       interpolate regexes into	larger regexes and not have to worry about the
       capture groups being renumbered.

       You can dispense	with numbers altogether	and create named capture
       groups.	The notation is	"(?<name_...)" to declare and "\g{name}" to
       reference.  (To be compatible with .Net regular expressions, "\g{name}"
       may also	be written as "\k{name}", "\k<name_" or	"\k'name'".)  name
       must not	begin with a number, nor contain hyphens.  When	different
       groups within the same pattern have the same name, any reference	to
       that name assumes the leftmost defined group.  Named groups count in
       absolute	and relative numbering,	and so can also	be referred to by
       those numbers.  (It's possible to do things with	named capture groups
       that would otherwise require "(??{})".)

       Capture group contents are dynamically scoped and available to you
       outside the pattern until the end of the	enclosing block	or until the
       next successful match, whichever	comes first.  (See "Compound
       Statements" in perlsyn.)	 You can refer to them by absolute number
       (using "$1" instead of "\g1", etc); or by name via the "%+" hash, using
       "$+{name}".

       Braces are required in referring	to named capture groups, but are
       optional	for absolute or	relative numbered ones.	 Braces	are safer when
       creating	a regex	by concatenating smaller strings.  For example if you
       have "qr/$a$b/",	and $a contained "\g1",	and $b contained "37", you
       would get "/\g137/" which is probably not what you intended.

       The "\g"	and "\k" notations were	introduced in Perl 5.10.0.  Prior to
       that there were no named	nor relative numbered capture groups.
       Absolute	numbered groups	were referred to using "\1", "\2", etc., and
       this notation is	still accepted (and likely always will be).  But it
       leads to	some ambiguities if there are more than	9 capture groups, as
       "\10" could mean	either the tenth capture group,	or the character whose
       ordinal in octal	is 010 (a backspace in ASCII).	Perl resolves this
       ambiguity by interpreting "\10" as a backreference only if at least 10
       left parentheses	have opened before it.	Likewise "\11" is a
       backreference only if at	least 11 left parentheses have opened before
       it.  And	so on.	"\1" through "\9" are always interpreted as
       backreferences.	There are several examples below that illustrate these
       perils.	You can	avoid the ambiguity by always using "\g{}" or "\g" if
       you mean	capturing groups; and for octal	constants always using "\o{}",
       or for "\077" and below,	using 3	digits padded with leading zeros,
       since a leading zero implies an octal constant.

       The "\digit" notation also works	in certain circumstances outside the
       pattern.	 See "Warning on \1 Instead of $1" below for details.

       Examples:

	   s/^([^ ]*) *([^ ]*)/$2 $1/;	   # swap first	two words

	   /(.)\g1/			   # find first	doubled	char
		and print "'$1'	is the first doubled character\n";

	   /(?<char>.)\k<char>/		   # ... a different way
		and print "'$+{char}' is the first doubled character\n";

	   /(?'char'.)\g1/		   # ... mix and match
		and print "'$1'	is the first doubled character\n";

	   if (/Time: (..):(..):(..)/) {   # parse out values
	       $hours =	$1;
	       $minutes	= $2;
	       $seconds	= $3;
	   }

	   /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/   # \g10 is a backreference
	   /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/    # \10 is	octal
	   /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/  # \10 is	a backreference
	   /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal

	   $a =	'(.)\1';	# Creates problems when	concatenated.
	   $b =	'(.)\g{1}';	# Avoids the problems.
	   "aa"	=~ /${a}/;	# True
	   "aa"	=~ /${b}/;	# True
	   "aa0" =~ /${a}0/;	# False!
	   "aa0" =~ /${b}0/;	# True
	   "aa\x08" =~ /${a}0/;	 # True!
	   "aa\x08" =~ /${b}0/;	 # False

       Several special variables also refer back to portions of	the previous
       match.  $+ returns whatever the last bracket match matched.  $& returns
       the entire matched string.  (At one point $0 did	also, but now it
       returns the name	of the program.)  "$`" returns everything before the
       matched string.	"$'" returns everything	after the matched string. And
       $^N contains whatever was matched by the	most-recently closed group
       (submatch). $^N can be used in extended patterns	(see below), for
       example to assign a submatch to a variable.

       These special variables,	like the "%+" hash and the numbered match
       variables ($1, $2, $3, etc.) are	dynamically scoped until the end of
       the enclosing block or until the	next successful	match, whichever comes
       first.  (See "Compound Statements" in perlsyn.)

       NOTE: Failed matches in Perl do not reset the match variables, which
       makes it	easier to write	code that tests	for a series of	more specific
       cases and remembers the best match.

       WARNING:	If your	code is	to run on Perl 5.16 or earlier,	beware that
       once Perl sees that you need one	of $&, "$`", or	"$'" anywhere in the
       program,	it has to provide them for every pattern match.	 This may
       substantially slow your program.

       Perl uses the same mechanism to produce $1, $2, etc, so you also	pay a
       price for each pattern that contains capturing parentheses.  (To	avoid
       this cost while retaining the grouping behaviour, use the extended
       regular expression "(?: ... )" instead.)	 But if	you never use $&, "$`"
       or "$'",	then patterns without capturing	parentheses will not be
       penalized.  So avoid $&,	"$'", and "$`" if you can, but if you can't
       (and some algorithms really appreciate them), once you've used them
       once, use them at will, because you've already paid the price.

       Perl 5.16 introduced a slightly more efficient mechanism	that notes
       separately whether each of "$`",	$&, and	"$'" have been seen, and thus
       may only	need to	copy part of the string.  Perl 5.20 introduced a much
       more efficient copy-on-write mechanism which eliminates any slowdown.

       As another workaround for this problem, Perl 5.10.0 introduced
       "${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}",	which are equivalent
       to "$`",	$& and "$'", except that they are only guaranteed to be
       defined after a successful match	that was executed with the "/p"
       (preserve) modifier.  The use of	these variables	incurs no global
       performance penalty, unlike their punctuation character equivalents,
       however at the trade-off	that you have to tell perl when	you want to
       use them.  As of	Perl 5.20, these three variables are equivalent	to
       "$`", $&	and "$'", and "/p" is ignored.

   Quoting metacharacters
       Backslashed metacharacters in Perl are alphanumeric, such as "\b",
       "\w", "\n".  Unlike some	other regular expression languages, there are
       no backslashed symbols that aren't alphanumeric.	 So anything that
       looks like "\\",	"\(", "\)", "\[", "\]",	"\{", or "\}" is always
       interpreted as a	literal	character, not a metacharacter.	 This was once
       used in a common	idiom to disable or quote the special meanings of
       regular expression metacharacters in a string that you want to use for
       a pattern. Simply quote all non-"word" characters:

	   $pattern =~ s/(\W)/\\$1/g;

       (If "use	locale"	is set,	then this depends on the current locale.)
       Today it	is more	common to use the "quotemeta()"	function or the	"\Q"
       metaquoting escape sequence to disable all metacharacters' special
       meanings	like this:

	   /$unquoted\Q$quoted\E$unquoted/

       Beware that if you put literal backslashes (those not inside
       interpolated variables) between "\Q" and	"\E", double-quotish backslash
       interpolation may lead to confusing results.  If	you need to use
       literal backslashes within "\Q...\E", consult "Gory details of parsing
       quoted constructs" in perlop.

       "quotemeta()" and "\Q" are fully	described in "quotemeta" in perlfunc.

   Extended Patterns
       Perl also defines a consistent extension	syntax for features not	found
       in standard tools like awk and lex.  The	syntax for most	of these is a
       pair of parentheses with	a question mark	as the first thing within the
       parentheses.  The character after the question mark indicates the
       extension.

       A question mark was chosen for this and for the minimal-matching
       construct because 1) question marks are rare in older regular
       expressions, and	2) whenever you	see one, you should stop and
       "question" exactly what is going	on.  That's psychology....

       "(?#text)"
	   A comment.  The text	is ignored.  Note that Perl closes the comment
	   as soon as it sees a	")", so	there is no way	to put a literal ")"
	   in the comment.  The	pattern's closing delimiter must be escaped by
	   a backslash if it appears in	the comment.

	   See "/x" for	another	way to have comments in	patterns.

	   Note	that a comment can go just about anywhere, except in the
	   middle of an	escape sequence.   Examples:

	    qr/foo(?#comment)bar/'  # Matches 'foobar'

	    # The pattern below	matches	'abcd',	'abccd', or 'abcccd'
	    qr/abc(?#comment between literal and its quantifier){1,3}d/

	    # The pattern below	generates a syntax error, because the '\p' must
	    # be followed immediately by a '{'.
	    qr/\p(?#comment between \p and its property	name){Any}/

	    # The pattern below	generates a syntax error, because the initial
	    # '\(' is a	literal	opening	parenthesis, and so there is nothing
	    # for the  closing ')' to match
	    qr/\(?#the backslash means this isn't a comment)p{Any}/

       "(?adlupimnsx-imnsx)"
       "(?^alupimnsx)"
	   One or more embedded	pattern-match modifiers, to be turned on (or
	   turned off if preceded by "-") for the remainder of the pattern or
	   the remainder of the	enclosing pattern group	(if any).

	   This	is particularly	useful for dynamically-generated patterns,
	   such	as those read in from a	configuration file, taken from an
	   argument, or	specified in a table somewhere.	 Consider the case
	   where some patterns want to be case-sensitive and some do not:  The
	   case-insensitive ones merely	need to	include	"(?i)" at the front of
	   the pattern.	 For example:

	       $pattern	= "foobar";
	       if ( /$pattern/i	) { }

	       # more flexible:

	       $pattern	= "(?i)foobar";
	       if ( /$pattern/ ) { }

	   These modifiers are restored	at the end of the enclosing group. For
	   example,

	       ( (?i) blah ) \s+ \g1

	   will	match "blah" in	any case, some spaces, and an exact (including
	   the case!)  repetition of the previous word,	assuming the "/x"
	   modifier, and no "/i" modifier outside this group.

	   These modifiers do not carry	over into named	subpatterns called in
	   the enclosing group.	In other words,	a pattern such as
	   "((?i)(?&NAME))" does not change the	case-sensitivity of the	"NAME"
	   pattern.

	   A modifier is overridden by later occurrences of this construct in
	   the same scope containing the same modifier,	so that

	       /((?im)foo(?-m)bar)/

	   matches all of "foobar" case	insensitively, but uses	"/m" rules for
	   only	the "foo" portion.  The	"a" flag overrides "aa"	as well;
	   likewise "aa" overrides "a".	 The same goes for "x" and "xx".
	   Hence, in

	       /(?-x)foo/xx

	   both	"/x" and "/xx" are turned off during matching "foo".  And in

	       /(?x)foo/x

	   "/x"	but NOT	"/xx" is turned	on for matching	"foo".	(One might
	   mistakenly think that since the inner "(?x)"	is already in the
	   scope of "/x", that the result would	effectively be the sum of
	   them, yielding "/xx".  It doesn't work that way.)  Similarly, doing
	   something like "(?xx-x)foo" turns off all "x" behavior for matching
	   "foo", it is	not that you subtract 1	"x" from 2 to get 1 "x"
	   remaining.

	   Any of these	modifiers can be set to	apply globally to all regular
	   expressions compiled	within the scope of a "use re".	 See "'/flags'
	   mode" in re.

	   Starting in Perl 5.14, a "^"	(caret or circumflex accent)
	   immediately after the "?" is	a shorthand equivalent to "d-imnsx".
	   Flags (except "d") may follow the caret to override it.  But	a
	   minus sign is not legal with	it.

	   Note	that the "a", "d", "l",	"p", and "u" modifiers are special in
	   that	they can only be enabled, not disabled,	and the	"a", "d", "l",
	   and "u" modifiers are mutually exclusive: specifying	one de-
	   specifies the others, and a maximum of one (or two "a"'s) may
	   appear in the construct.  Thus, for example,	"(?-p)"	will warn when
	   compiled under "use warnings"; "(?-d:...)" and "(?dl:...)" are
	   fatal errors.

	   Note	also that the "p" modifier is special in that its presence
	   anywhere in a pattern has a global effect.

       "(?:pattern)"
       "(?adluimnsx-imnsx:pattern)"
       "(?^aluimnsx:pattern)"
	   This	is for clustering, not capturing; it groups subexpressions
	   like	"()", but doesn't make backreferences as "()" does.  So

	       @fields = split(/\b(?:a|b|c)\b/)

	   matches the same field delimiters as

	       @fields = split(/\b(a|b|c)\b/)

	   but doesn't spit out	the delimiters themselves as extra fields
	   (even though	that's the behaviour of	"split"	in perlfunc when its
	   pattern contains capturing groups).	It's also cheaper not to
	   capture characters if you don't need	to.

	   Any letters between "?" and ":" act as flags	modifiers as with
	   "(?adluimnsx-imnsx)".  For example,

	       /(?s-i:more.*than).*million/i

	   is equivalent to the	more verbose

	       /(?:(?s-i)more.*than).*million/i

	   Note	that any "()" constructs enclosed within this one will still
	   capture unless the "/n" modifier is in effect.

	   Like	the "(?adlupimnsx-imnsx)" construct, "aa" and "a" override
	   each	other, as do "xx" and "x".  They are not additive.  So,	doing
	   something like "(?xx-x:foo)"	turns off all "x" behavior for
	   matching "foo".

	   Starting in Perl 5.14, a "^"	(caret or circumflex accent)
	   immediately after the "?" is	a shorthand equivalent to "d-imnsx".
	   Any positive	flags (except "d") may follow the caret, so

	       (?^x:foo)

	   is equivalent to

	       (?x-imns:foo)

	   The caret tells Perl	that this cluster doesn't inherit the flags of
	   any surrounding pattern, but	uses the system	defaults ("d-imnsx"),
	   modified by any flags specified.

	   The caret allows for	simpler	stringification	of compiled regular
	   expressions.	 These look like

	       (?^:pattern)

	   with	any non-default	flags appearing	between	the caret and the
	   colon.  A test that looks at	such stringification thus doesn't need
	   to have the system default flags hard-coded in it, just the caret.
	   If new flags	are added to Perl, the meaning of the caret's
	   expansion will change to include the	default	for those flags, so
	   the test will still work, unchanged.

	   Specifying a	negative flag after the	caret is an error, as the flag
	   is redundant.

	   Mnemonic for	"(?^...)":  A fresh beginning since the	usual use of a
	   caret is to match at	the beginning.

       "(?|pattern)"
	   This	is the "branch reset" pattern, which has the special property
	   that	the capture groups are numbered	from the same starting point
	   in each alternation branch. It is available starting	from perl
	   5.10.0.

	   Capture groups are numbered from left to right, but inside this
	   construct the numbering is restarted	for each branch.

	   The numbering within	each branch will be as normal, and any groups
	   following this construct will be numbered as	though the construct
	   contained only one branch, that being the one with the most capture
	   groups in it.

	   This	construct is useful when you want to capture one of a number
	   of alternative matches.

	   Consider the	following pattern.  The	numbers	underneath show	in
	   which group the captured content will be stored.

	       # before	 ---------------branch-reset-----------	after
	       / ( a )	(?| x (	y ) z |	(p (q) r) | (t)	u (v) )	( z ) /x
	       # 1	      2		2  3	    2	  3	4

	   Be careful when using the branch reset pattern in combination with
	   named captures. Named captures are implemented as being aliases to
	   numbered groups holding the captures, and that interferes with the
	   implementation of the branch	reset pattern. If you are using	named
	   captures in a branch	reset pattern, it's best to use	the same
	   names, in the same order, in	each of	the alternations:

	      /(?|  (?<a> x ) (?<b> y )
		 |  (?<a> z ) (?<b> w )) /x

	   Not doing so	may lead to surprises:

	     "12" =~ /(?| (?<a>	\d+ ) |	(?<b> \D+))/x;
	     say $+{a};	   # Prints '12'
	     say $+{b};	   # *Also* prints '12'.

	   The problem here is that both the group named "a" and the group
	   named "b" are aliases for the group belonging to $1.

       Lookaround Assertions
	   Lookaround assertions are zero-width	patterns which match a
	   specific pattern without including it in $&.	Positive assertions
	   match when their subpattern matches,	negative assertions match when
	   their subpattern fails. Lookbehind matches text up to the current
	   match position, lookahead matches text following the	current	match
	   position.

	   "(?=pattern)"
	       A zero-width positive lookahead assertion.  For example,
	       "/\w+(?=\t)/" matches a word followed by	a tab, without
	       including the tab in $&.

	   "(?!pattern)"
	       A zero-width negative lookahead assertion.  For example
	       "/foo(?!bar)/" matches any occurrence of	"foo" that isn't
	       followed	by "bar".  Note	however	that lookahead and lookbehind
	       are NOT the same	thing.	You cannot use this for	lookbehind.

	       If you are looking for a	"bar" that isn't preceded by a "foo",
	       "/(?!foo)bar/" will not do what you want.  That's because the
	       "(?!foo)" is just saying	that the next thing cannot be
	       "foo"--and it's not, it's a "bar", so "foobar" will match.  Use
	       lookbehind instead (see below).

	   "(?<=pattern)"
	   "\K"
	       A zero-width positive lookbehind	assertion.  For	example,
	       "/(?<=\t)\w+/" matches a	word that follows a tab, without
	       including the tab in $&.	 Works only for	fixed-width
	       lookbehind.

	       There is	a special form of this construct, called "\K"
	       (available since	Perl 5.10.0), which causes the regex engine to
	       "keep" everything it had	matched	prior to the "\K" and not
	       include it in $&. This effectively provides variable-length
	       lookbehind. The use of "\K" inside of another lookaround
	       assertion is allowed, but the behaviour is currently not	well
	       defined.

	       For various reasons "\K"	may be significantly more efficient
	       than the	equivalent "(?<=...)" construct, and it	is especially
	       useful in situations where you want to efficiently remove
	       something following something else in a string. For instance

		 s/(foo)bar/$1/g;

	       can be rewritten	as the much more efficient

		 s/foo\Kbar//g;

	   "(?<!pattern)"
	       A zero-width negative lookbehind	assertion.  For	example
	       "/(?<!bar)foo/" matches any occurrence of "foo" that does not
	       follow "bar".  Works only for fixed-width lookbehind.

       "(?<NAME>pattern)"
       "(?'NAME'pattern)"
	   A named capture group. Identical in every respect to	normal
	   capturing parentheses "()" but for the additional fact that the
	   group can be	referred to by name in various regular expression
	   constructs (like "\g{NAME}")	and can	be accessed by name after a
	   successful match via	"%+" or	"%-". See perlvar for more details on
	   the "%+" and	"%-" hashes.

	   If multiple distinct	capture	groups have the	same name then the
	   $+{NAME} will refer to the leftmost defined group in	the match.

	   The forms "(?'NAME'pattern)"	and "(?<NAME>pattern)" are equivalent.

	   NOTE: While the notation of this construct is the same as the
	   similar function in .NET regexes, the behavior is not. In Perl the
	   groups are numbered sequentially regardless of being	named or not.
	   Thus	in the pattern

	     /(x)(?<foo>y)(z)/

	   $+{foo} will	be the same as $2, and $3 will contain 'z' instead of
	   the opposite	which is what a	.NET regex hacker might	expect.

	   Currently NAME is restricted	to simple identifiers only.  In	other
	   words, it must match	"/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
	   extension (see utf8), though	it isn't extended by the locale	(see
	   perllocale).

	   NOTE: In order to make things easier	for programmers	with
	   experience with the Python or PCRE regex engines, the pattern
	   "(?P<NAME>pattern)" may be used instead of "(?<NAME>pattern)";
	   however this	form does not support the use of single	quotes as a
	   delimiter for the name.

       "\k<NAME>"
       "\k'NAME'"
	   Named backreference.	Similar	to numeric backreferences, except that
	   the group is	designated by name and not number. If multiple groups
	   have	the same name then it refers to	the leftmost defined group in
	   the current match.

	   It is an error to refer to a	name not defined by a "(?<NAME>)"
	   earlier in the pattern.

	   Both	forms are equivalent.

	   NOTE: In order to make things easier	for programmers	with
	   experience with the Python or PCRE regex engines, the pattern
	   "(?P=NAME)" may be used instead of "\k<NAME>".

       "(?{ code })"
	   WARNING: Using this feature safely requires that you	understand its
	   limitations.	 Code executed that has	side effects may not perform
	   identically from version to version due to the effect of future
	   optimisations in the	regex engine.  For more	information on this,
	   see "Embedded Code Execution	Frequency".

	   This	zero-width assertion executes any embedded Perl	code.  It
	   always succeeds, and	its return value is set	as $^R.

	   In literal patterns,	the code is parsed at the same time as the
	   surrounding code. While within the pattern, control is passed
	   temporarily back to the perl	parser,	until the logically-balancing
	   closing brace is encountered. This is similar to the	way that an
	   array index expression in a literal string is handled, for example

	       "abc$array[ 1 + f('[') +	g()]def"

	   In particular, braces do not	need to	be balanced:

	       s/abc(?{	f('{');	})/def/

	   Even	in a pattern that is interpolated and compiled at run-time,
	   literal code	blocks will be compiled	once, at perl compile time;
	   the following prints	"ABCD":

	       print "D";
	       my $qr =	qr/(?{ BEGIN { print "A" } })/;
	       my $foo = "foo";
	       /$foo$qr(?{ BEGIN { print "B" } })/;
	       BEGIN { print "C" }

	   In patterns where the text of the code is derived from run-time
	   information rather than appearing literally in a source code
	   /pattern/, the code is compiled at the same time that the pattern
	   is compiled,	and for	reasons	of security, "use re 'eval'" must be
	   in scope. This is to	stop user-supplied patterns containing code
	   snippets from being executable.

	   In situations where you need	to enable this with "use re 'eval'",
	   you should also have	taint checking enabled.	 Better	yet, use the
	   carefully constrained evaluation within a Safe compartment.	See
	   perlsec for details about both these	mechanisms.

	   From	the viewpoint of parsing, lexical variable scope and closures,

	       /AAA(?{ BBB })CCC/

	   behaves approximately like

	       /AAA/ &&	do { BBB } && /CCC/

	   Similarly,

	       qr/AAA(?{ BBB })CCC/

	   behaves approximately like

	       sub { /AAA/ && do { BBB } && /CCC/ }

	   In particular:

	       { my $i = 1; $r = qr/(?{	print $i })/ }
	       my $i = 2;
	       /$r/; # prints "1"

	   Inside a "(?{...})" block, $_ refers	to the string the regular
	   expression is matching against. You can also	use "pos()" to know
	   what	is the current position	of matching within this	string.

	   The code block introduces a new scope from the perspective of
	   lexical variable declarations, but not from the perspective of
	   "local" and similar localizing behaviours. So later code blocks
	   within the same pattern will	still see the values which were
	   localized in	earlier	blocks.	 These accumulated localizations are
	   undone either at the	end of a successful match, or if the assertion
	   is backtracked (compare "Backtracking"). For	example,

	     $_	= 'a' x	8;
	     m<
		(?{ $cnt = 0 })		      #	Initialize $cnt.
		(
		  a
		  (?{
		      local $cnt = $cnt	+ 1;  #	Update $cnt,
					      #	backtracking-safe.
		  })
		)*
		aaaa
		(?{ $res = $cnt	})	      #	On success copy	to
					      #	non-localized location.
	      >x;

	   will	initially increment $cnt up to 8; then during backtracking,
	   its value will be unwound back to 4,	which is the value assigned to
	   $res.  At the end of	the regex execution, $cnt will be wound	back
	   to its initial value	of 0.

	   This	assertion may be used as the condition in a

	       (?(condition)yes-pattern|no-pattern)

	   switch.  If not used	in this	way, the result	of evaluation of
	   "code" is put into the special variable $^R.	 This happens
	   immediately,	so $^R can be used from	other "(?{ code	})" assertions
	   inside the same regular expression.

	   The assignment to $^R above is properly localized, so the old value
	   of $^R is restored if the assertion is backtracked; compare
	   "Backtracking".

	   Note	that the special variable $^N  is particularly useful with
	   code	blocks to capture the results of submatches in variables
	   without having to keep track	of the number of nested	parentheses.
	   For example:

	     $_	= "The brown fox jumps over the	lazy dog";
	     /the (\S+)(?{ $color = $^N	}) (\S+)(?{ $animal = $^N })/i;
	     print "color = $color, animal = $animal\n";

       "(??{ code })"
	   WARNING: Using this feature safely requires that you	understand its
	   limitations.	 Code executed that has	side effects may not perform
	   identically from version to version due to the effect of future
	   optimisations in the	regex engine.  For more	information on this,
	   see "Embedded Code Execution	Frequency".

	   This	is a "postponed" regular subexpression.	 It behaves in exactly
	   the same way	as a "(?{ code })" code	block as described above,
	   except that its return value, rather	than being assigned to $^R, is
	   treated as a	pattern, compiled if it's a string (or used as-is if
	   its a qr// object), then matched as if it were inserted instead of
	   this	construct.

	   During the matching of this sub-pattern, it has its own set of
	   captures which are valid during the sub-match, but are discarded
	   once	control	returns	to the main pattern. For example, the
	   following matches, with the inner pattern capturing "B" and
	   matching "BB", while	the outer pattern captures "A";

	       my $inner = '(.)\1';
	       "ABBA" =~ /^(.)(??{ $inner })\1/;
	       print $1; # prints "A";

	   Note	that this means	that  there is no way for the inner pattern to
	   refer to a capture group defined outside.  (The code	block itself
	   can use $1, etc., to	refer to the enclosing pattern's capture
	   groups.)  Thus, although

	       ('a' x 100)=~/(??{'(.)' x 100})/

	   will	match, it will not set $1 on exit.

	   The following pattern matches a parenthesized group:

	    $re	= qr{
		       \(
		       (?:
			  (?> [^()]+ )	# Non-parens without backtracking
			|
			  (??{ $re })	# Group	with matching parens
		       )*
		       \)
		    }x;

	   See also "(?PARNO)" for a different,	more efficient way to
	   accomplish the same task.

	   Executing a postponed regular expression too	many times without
	   consuming any input string will also	result in a fatal error.  The
	   depth at which that happens is compiled into	perl, so it can	be
	   changed with	a custom build.

       "(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
	   Recursive subpattern. Treat the contents of a given capture buffer
	   in the current pattern as an	independent subpattern and attempt to
	   match it at the current position in the string. Information about
	   capture state from the caller for things like backreferences	is
	   available to	the subpattern,	but capture buffers set	by the
	   subpattern are not visible to the caller.

	   Similar to "(??{ code })" except that it does not involve executing
	   any code or potentially compiling a returned	pattern	string;
	   instead it treats the part of the current pattern contained within
	   a specified capture group as	an independent pattern that must match
	   at the current position. Also different is the treatment of capture
	   buffers, unlike "(??{ code })" recursive patterns have access to
	   their caller's match	state, so one can use backreferences safely.

	   PARNO is a sequence of digits (not starting with 0) whose value
	   reflects the	paren-number of	the capture group to recurse to.
	   "(?R)" recurses to the beginning of the whole pattern. "(?0)" is an
	   alternate syntax for	"(?R)".	If PARNO is preceded by	a plus or
	   minus sign then it is assumed to be relative, with negative numbers
	   indicating preceding	capture	groups and positive ones following.
	   Thus	"(?-1)"	refers to the most recently declared group, and
	   "(?+1)" indicates the next group to be declared.  Note that the
	   counting for	relative recursion differs from	that of	relative
	   backreferences, in that with	recursion unclosed groups are
	   included.

	   The following pattern matches a function "foo()" which may contain
	   balanced parentheses	as the argument.

	     $re = qr{ (		   # paren group 1 (full function)
			 foo
			 (		   # paren group 2 (parens)
			   \(
			     (		   # paren group 3 (contents of	parens)
			     (?:
			      (?> [^()]+ ) # Non-parens	without	backtracking
			     |
			      (?2)	   # Recurse to	start of paren group 2
			     )*
			     )
			   \)
			 )
		       )
		     }x;

	   If the pattern was used as follows

	       'foo(bar(baz)+baz(bop))'=~/$re/
		   and print "\$1 = $1\n",
			     "\$2 = $2\n",
			     "\$3 = $3\n";

	   the output produced should be the following:

	       $1 = foo(bar(baz)+baz(bop))
	       $2 = (bar(baz)+baz(bop))
	       $3 = bar(baz)+baz(bop)

	   If there is no corresponding	capture	group defined, then it is a
	   fatal error.	 Recursing deeply without consuming any	input string
	   will	also result in a fatal error.  The depth at which that happens
	   is compiled into perl, so it	can be changed with a custom build.

	   The following shows how using negative indexing can make it easier
	   to embed recursive patterns inside of a "qr//" construct for	later
	   use:

	       my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
	       if (/foo	$parens	\s+ \+ \s+ bar $parens/x) {
		  # do something here...
	       }

	   Note	that this pattern does not behave the same way as the
	   equivalent PCRE or Python construct of the same form. In Perl you
	   can backtrack into a	recursed group,	in PCRE	and Python the
	   recursed into group is treated as atomic. Also, modifiers are
	   resolved at compile time, so	constructs like	"(?i:(?1))" or
	   "(?:(?i)(?1))" do not affect	how the	sub-pattern will be processed.

       "(?&NAME)"
	   Recurse to a	named subpattern. Identical to "(?PARNO)" except that
	   the parenthesis to recurse to is determined by name.	If multiple
	   parentheses have the	same name, then	it recurses to the leftmost.

	   It is an error to refer to a	name that is not declared somewhere in
	   the pattern.

	   NOTE: In order to make things easier	for programmers	with
	   experience with the Python or PCRE regex engines the	pattern
	   "(?P>NAME)" may be used instead of "(?&NAME)".

       "(?(condition)yes-pattern|no-pattern)"
       "(?(condition)yes-pattern)"
	   Conditional expression. Matches "yes-pattern" if "condition"	yields
	   a true value, matches "no-pattern" otherwise. A missing pattern
	   always matches.

	   "(condition)" should	be one of:

	   an integer in parentheses
	       (which is valid if the corresponding pair of parentheses
	       matched);

	   a lookahead/lookbehind/evaluate zero-width assertion;
	   a name in angle brackets or single quotes
	       (which is valid if a group with the given name matched);

	   the special symbol "(R)"
	       (true when evaluated inside of recursion	or eval).
	       Additionally the	"R" may	be followed by a number, (which	will
	       be true when evaluated when recursing inside of the appropriate
	       group), or by &NAME, in which case it will be true only when
	       evaluated during	recursion in the named group.

	   Here's a summary of the possible predicates:

	   "(1)" "(2)" ...
	       Checks if the numbered capturing	group has matched something.
	       Full syntax: "(?(1)then|else)"

	   "(<NAME>)" "('NAME')"
	       Checks if a group with the given	name has matched something.
	       Full syntax: "(?(<name>)then|else)"

	   "(?=...)" "(?!...)" "(?<=...)" "(?<!...)"
	       Checks whether the pattern matches (or does not match, for the
	       "!"  variants).	Full syntax: "(?(?=lookahead)then|else)"

	   "(?{	CODE })"
	       Treats the return value of the code block as the	condition.
	       Full syntax: "(?(?{ code	})then|else)"

	   "(R)"
	       Checks if the expression	has been evaluated inside of
	       recursion.  Full	syntax:	"(?(R)then|else)"

	   "(R1)" "(R2)" ...
	       Checks if the expression	has been evaluated while executing
	       directly	inside of the n-th capture group. This check is	the
	       regex equivalent	of

		 if ((caller(0))[3] eq 'subname') { ...	}

	       In other	words, it does not check the full recursion stack.

	       Full syntax: "(?(R1)then|else)"

	   "(R&NAME)"
	       Similar to "(R1)", this predicate checks	to see if we're
	       executing directly inside of the	leftmost group with a given
	       name (this is the same logic used by "(?&NAME)" to
	       disambiguate). It does not check	the full stack,	but only the
	       name of the innermost active recursion.	Full syntax:
	       "(?(R&name)then|else)"

	   "(DEFINE)"
	       In this case, the yes-pattern is	never directly executed, and
	       no no-pattern is	allowed. Similar in spirit to "(?{0})" but
	       more efficient.	See below for details.	Full syntax:
	       "(?(DEFINE)definitions...)"

	   For example:

	       m{ ( \( )?
		  [^()]+
		  (?(1)	\) )
		}x

	   matches a chunk of non-parentheses, possibly	included in
	   parentheses themselves.

	   A special form is the "(DEFINE)" predicate, which never executes
	   its yes-pattern directly, and does not allow	a no-pattern. This
	   allows one to define	subpatterns which will be executed only	by the
	   recursion mechanism.	 This way, you can define a set	of regular
	   expression rules that can be	bundled	into any pattern you choose.

	   It is recommended that for this usage you put the DEFINE block at
	   the end of the pattern, and that you	name any subpatterns defined
	   within it.

	   Also, it's worth noting that	patterns defined this way probably
	   will	not be as efficient, as	the optimizer is not very clever about
	   handling them.

	   An example of how this might	be used	is as follows:

	     /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
	      (?(DEFINE)
		(?<NAME_PAT>....)
		(?<ADDRESS_PAT>....)
	      )/x

	   Note	that capture groups matched inside of recursion	are not
	   accessible after the	recursion returns, so the extra	layer of
	   capturing groups is necessary. Thus $+{NAME_PAT} would not be
	   defined even	though $+{NAME}	would be.

	   Finally, keep in mind that subpatterns created inside a DEFINE
	   block count towards the absolute and	relative number	of captures,
	   so this:

	       my @captures = "a" =~ /(.)		   # First capture
				      (?(DEFINE)
					  (?<EXAMPLE> 1	)  # Second capture
				      )/x;
	       say scalar @captures;

	   Will	output 2, not 1. This is particularly important	if you intend
	   to compile the definitions with the "qr//" operator,	and later
	   interpolate them in another pattern.

       "(?>pattern)"
	   An "independent" subexpression, one which matches the substring
	   that	a standalone "pattern" would match if anchored at the given
	   position, and it matches nothing other than this substring.	This
	   construct is	useful for optimizations of what would otherwise be
	   "eternal" matches, because it will not backtrack (see
	   "Backtracking").  It	may also be useful in places where the "grab
	   all you can,	and do not give	anything back" semantic	is desirable.

	   For example:	"^(?>a*)ab" will never match, since "(?>a*)" (anchored
	   at the beginning of string, as above) will match all	characters "a"
	   at the beginning of string, leaving no "a" for "ab" to match.  In
	   contrast, "a*ab" will match the same	as "a+b", since	the match of
	   the subgroup	"a*" is	influenced by the following group "ab" (see
	   "Backtracking").  In	particular, "a*" inside	"a*ab" will match
	   fewer characters than a standalone "a*", since this makes the tail
	   match.

	   "(?>pattern)" does not disable backtracking altogether once it has
	   matched. It is still	possible to backtrack past the construct, but
	   not into it.	So "((?>a*)|(?>b*))ar" will still match	"bar".

	   An effect similar to	"(?>pattern)" may be achieved by writing
	   "(?=(pattern))\g{-1}".  This	matches	the same substring as a
	   standalone "a+", and	the following "\g{-1}" eats the	matched
	   string; it therefore	makes a	zero-length assertion into an analogue
	   of "(?>...)".  (The difference between these	two constructs is that
	   the second one uses a capturing group, thus shifting	ordinals of
	   backreferences in the rest of a regular expression.)

	   Consider this pattern:

	       m{ \(
		     (
		       [^()]+		# x+
		     |
		       \( [^()]* \)
		     )+
		  \)
		}x

	   That	will efficiently match a nonempty group	with matching
	   parentheses two levels deep or less.	 However, if there is no such
	   group, it will take virtually forever on a long string.  That's
	   because there are so	many different ways to split a long string
	   into	several	substrings.  This is what "(.+)+" is doing, and
	   "(.+)+" is similar to a subpattern of the above pattern.  Consider
	   how the pattern above detects no-match on "((()aaaaaaaaaaaaaaaaaa"
	   in several seconds, but that	each extra letter doubles this time.
	   This	exponential performance	will make it appear that your program
	   has hung.  However, a tiny change to	this pattern

	       m{ \(
		     (
		       (?> [^()]+ )	   # change x+ above to	(?> x+ )
		     |
		       \( [^()]* \)
		     )+
		  \)
		}x

	   which uses "(?>...)"	matches	exactly	when the one above does
	   (verifying this yourself would be a productive exercise), but
	   finishes in a fourth	the time when used on a	similar	string with
	   1000000 "a"s.  Be aware, however, that, when	this construct is
	   followed by a quantifier, it	currently triggers a warning message
	   under the "use warnings" pragma or -w switch	saying it "matches
	   null	string many times in regex".

	   On simple groups, such as the pattern "(?> [^()]+ )", a comparable
	   effect may be achieved by negative lookahead, as in "[^()]+ (?!
	   [^()] )".  This was only 4 times slower on a	string with 1000000
	   "a"s.

	   The "grab all you can, and do not give anything back" semantic is
	   desirable in	many situations	where on the first sight a simple
	   "()*" looks like the	correct	solution.  Suppose we parse text with
	   comments being delimited by "#" followed by some optional
	   (horizontal)	whitespace.  Contrary to its appearance, "#[ \t]*" is
	   not the correct subexpression to match the comment delimiter,
	   because it may "give	up" some whitespace if the remainder of	the
	   pattern can be made to match	that way.  The correct answer is
	   either one of these:

	       (?>#[ \t]*)
	       #[ \t]*(?![ \t])

	   For example,	to grab	non-empty comments into	$1, one	should use
	   either one of these:

	       / (?> \#	[ \t]* ) (	  .+ ) /x;
	       /     \#	[ \t]*	 ( [^ \t] .* ) /x;

	   Which one you pick depends on which of these	expressions better
	   reflects the	above specification of comments.

	   In some literature this construct is	called "atomic matching" or
	   "possessive matching".

	   Possessive quantifiers are equivalent to putting the	item they are
	   applied to inside of	one of these constructs. The following
	   equivalences	apply:

	       Quantifier Form	   Bracketing Form
	       ---------------	   ---------------
	       PAT*+		   (?>PAT*)
	       PAT++		   (?>PAT+)
	       PAT?+		   (?>PAT?)
	       PAT{min,max}+	   (?>PAT{min,max})

       "(?[ ])"
	   See "Extended Bracketed Character Classes" in perlrecharclass.

	   Note	that this feature is currently experimental; using it yields a
	   warning in the "experimental::regex_sets" category.

   Backtracking
       NOTE: This section presents an abstract approximation of	regular
       expression behavior.  For a more	rigorous (and complicated) view	of the
       rules involved in selecting a match among possible alternatives,	see
       "Combining RE Pieces".

       A fundamental feature of	regular	expression matching involves the
       notion called backtracking, which is currently used (when needed) by
       all regular non-possessive expression quantifiers, namely "*", "*?",
       "+", "+?", "{n,m}", and "{n,m}?".  Backtracking is often	optimized
       internally, but the general principle outlined here is valid.

       For a regular expression	to match, the entire regular expression	must
       match, not just part of it.  So if the beginning	of a pattern
       containing a quantifier succeeds	in a way that causes later parts in
       the pattern to fail, the	matching engine	backs up and recalculates the
       beginning part--that's why it's called backtracking.

       Here is an example of backtracking:  Let's say you want to find the
       word following "foo" in the string "Food	is on the foo table.":

	   $_ =	"Food is on the	foo table.";
	   if (	/\b(foo)\s+(\w+)/i ) {
	       print "$2 follows $1.\n";
	   }

       When the	match runs, the	first part of the regular expression
       ("\b(foo)") finds a possible match right	at the beginning of the
       string, and loads up $1 with "Foo".  However, as	soon as	the matching
       engine sees that	there's	no whitespace following	the "Foo" that it had
       saved in	$1, it realizes	its mistake and	starts over again one
       character after where it	had the	tentative match.  This time it goes
       all the way until the next occurrence of	"foo". The complete regular
       expression matches this time, and you get the expected output of	"table
       follows foo."

       Sometimes minimal matching can help a lot.  Imagine you'd like to match
       everything between "foo"	and "bar".  Initially, you write something
       like this:

	   $_ =	 "The food is under the	bar in the barn.";
	   if (	/foo(.*)bar/ ) {
	       print "got <$1>\n";
	   }

       Which perhaps unexpectedly yields:

	 got <d	is under the bar in the	>

       That's because ".*" was greedy, so you get everything between the first
       "foo" and the last "bar".  Here it's more effective to use minimal
       matching	to make	sure you get the text between a	"foo" and the first
       "bar" thereafter.

	   if (	/foo(.*?)bar/ )	{ print	"got <$1>\n" }
	 got <d	is under the >

       Here's another example. Let's say you'd like to match a number at the
       end of a	string,	and you	also want to keep the preceding	part of	the
       match.  So you write this:

	   $_ =	"I have	2 numbers: 53147";
	   if (	/(.*)(\d*)/ ) {				       # Wrong!
	       print "Beginning	is <$1>, number	is <$2>.\n";
	   }

       That won't work at all, because ".*" was	greedy and gobbled up the
       whole string. As	"\d*" can match	on an empty string the complete
       regular expression matched successfully.

	   Beginning is	<I have	2 numbers: 53147>, number is <>.

       Here are	some variants, most of which don't work:

	   $_ =	"I have	2 numbers: 53147";
	   @pats = qw{
	       (.*)(\d*)
	       (.*)(\d+)
	       (.*?)(\d*)
	       (.*?)(\d+)
	       (.*)(\d+)$
	       (.*?)(\d+)$
	       (.*)\b(\d+)$
	       (.*\D)(\d+)$
	   };

	   for $pat (@pats) {
	       printf "%-12s ",	$pat;
	       if ( /$pat/ ) {
		   print "<$1> <$2>\n";
	       } else {
		   print "FAIL\n";
	       }
	   }

       That will print out:

	   (.*)(\d*)	<I have	2 numbers: 53147> <>
	   (.*)(\d+)	<I have	2 numbers: 5314> <7>
	   (.*?)(\d*)	<> <>
	   (.*?)(\d+)	<I have	> <2>
	   (.*)(\d+)$	<I have	2 numbers: 5314> <7>
	   (.*?)(\d+)$	<I have	2 numbers: > <53147>
	   (.*)\b(\d+)$	<I have	2 numbers: > <53147>
	   (.*\D)(\d+)$	<I have	2 numbers: > <53147>

       As you see, this	can be a bit tricky.  It's important to	realize	that a
       regular expression is merely a set of assertions	that gives a
       definition of success.  There may be 0, 1, or several different ways
       that the	definition might succeed against a particular string.  And if
       there are multiple ways it might	succeed, you need to understand
       backtracking to know which variety of success you will achieve.

       When using lookahead assertions and negations, this can all get even
       trickier.  Imagine you'd	like to	find a sequence	of non-digits not
       followed	by "123".  You might try to write that as

	   $_ =	"ABC123";
	   if (	/^\D*(?!123)/ )	{		 # Wrong!
	       print "Yup, no 123 in $_\n";
	   }

       But that	isn't going to match; at least,	not the	way you're hoping.  It
       claims that there is no 123 in the string.  Here's a clearer picture of
       why that	pattern	matches, contrary to popular expectations:

	   $x =	'ABC123';
	   $y =	'ABC445';

	   print "1: got $1\n" if $x =~	/^(ABC)(?!123)/;
	   print "2: got $1\n" if $y =~	/^(ABC)(?!123)/;

	   print "3: got $1\n" if $x =~	/^(\D*)(?!123)/;
	   print "4: got $1\n" if $y =~	/^(\D*)(?!123)/;

       This prints

	   2: got ABC
	   3: got AB
	   4: got ABC

       You might have expected test 3 to fail because it seems to a more
       general purpose version of test 1.  The important difference between
       them is that test 3 contains a quantifier ("\D*") and so	can use
       backtracking, whereas test 1 will not.  What's happening	is that	you've
       asked "Is it true that at the start of $x, following 0 or more non-
       digits, you have	something that's not 123?"  If the pattern matcher had
       let "\D*" expand	to "ABC", this would have caused the whole pattern to
       fail.

       The search engine will initially	match "\D*" with "ABC".	 Then it will
       try to match "(?!123)" with "123", which	fails.	But because a
       quantifier ("\D*") has been used	in the regular expression, the search
       engine can backtrack and	retry the match	differently in the hope	of
       matching	the complete regular expression.

       The pattern really, really wants	to succeed, so it uses the standard
       pattern back-off-and-retry and lets "\D*" expand	to just	"AB" this
       time.  Now there's indeed something following "AB" that is not "123".
       It's "C123", which suffices.

       We can deal with	this by	using both an assertion	and a negation.	 We'll
       say that	the first part in $1 must be followed both by a	digit and by
       something that's	not "123".  Remember that the lookaheads are zero-
       width expressions--they only look, but don't consume any	of the string
       in their	match.	So rewriting this way produces what you'd expect; that
       is, case	5 will fail, but case 6	succeeds:

	   print "5: got $1\n" if $x =~	/^(\D*)(?=\d)(?!123)/;
	   print "6: got $1\n" if $y =~	/^(\D*)(?=\d)(?!123)/;

	   6: got ABC

       In other	words, the two zero-width assertions next to each other	work
       as though they're ANDed together, just as you'd use any built-in
       assertions:  "/^$/" matches only	if you're at the beginning of the line
       AND the end of the line simultaneously.	The deeper underlying truth is
       that juxtaposition in regular expressions always	means AND, except when
       you write an explicit OR	using the vertical bar.	 "/ab/"	means match
       "a" AND (then) match "b", although the attempted	matches	are made at
       different positions because "a" is not a	zero-width assertion, but a
       one-width assertion.

       WARNING:	Particularly complicated regular expressions can take
       exponential time	to solve because of the	immense	number of possible
       ways they can use backtracking to try for a match.  For example,
       without internal	optimizations done by the regular expression engine,
       this will take a	painfully long time to run:

	   'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/

       And if you used "*"'s in	the internal groups instead of limiting	them
       to 0 through 5 matches, then it would take forever--or until you	ran
       out of stack space.  Moreover, these internal optimizations are not
       always applicable.  For example,	if you put "{0,5}" instead of "*" on
       the external group, no current optimization is applicable, and the
       match takes a long time to finish.

       A powerful tool for optimizing such beasts is what is known as an
       "independent group", which does not backtrack (see ""(?>pattern)"").
       Note also that zero-length lookahead/lookbehind assertions will not
       backtrack to make the tail match, since they are	in "logical" context:
       only whether they match is considered relevant.	For an example where
       side-effects of lookahead might have influenced the following match,
       see ""(?>pattern)"".

   Special Backtracking	Control	Verbs
       These special patterns are generally of the form	"(*VERB:ARG)". Unless
       otherwise stated	the ARG	argument is optional; in some cases, it	is
       mandatory.

       Any pattern containing a	special	backtracking verb that allows an
       argument	has the	special	behaviour that when executed it	sets the
       current package's $REGERROR and $REGMARK	variables. When	doing so the
       following rules apply:

       On failure, the $REGERROR variable will be set to the ARG value of the
       verb pattern, if	the verb was involved in the failure of	the match. If
       the ARG part of the pattern was omitted,	then $REGERROR will be set to
       the name	of the last "(*MARK:NAME)" pattern executed, or	to TRUE	if
       there was none. Also, the $REGMARK variable will	be set to FALSE.

       On a successful match, the $REGERROR variable will be set to FALSE, and
       the $REGMARK variable will be set to the	name of	the last
       "(*MARK:NAME)" pattern executed.	 See the explanation for the
       "(*MARK:NAME)" verb below for more details.

       NOTE: $REGERROR and $REGMARK are	not magic variables like $1 and	most
       other regex-related variables. They are not local to a scope, nor
       readonly, but instead are volatile package variables similar to
       $AUTOLOAD.  They	are set	in the package containing the code that
       executed	the regex (rather than the one that compiled it, where those
       differ).	 If necessary, you can use "local" to localize changes to
       these variables to a specific scope before executing a regex.

       If a pattern does not contain a special backtracking verb that allows
       an argument, then $REGERROR and $REGMARK	are not	touched	at all.

       Verbs
	  "(*PRUNE)" "(*PRUNE:NAME)"
	      This zero-width pattern prunes the backtracking tree at the
	      current point when backtracked into on failure. Consider the
	      pattern "/A (*PRUNE) B/",	where A	and B are complex patterns.
	      Until the	"(*PRUNE)" verb	is reached, A may backtrack as
	      necessary	to match. Once it is reached, matching continues in B,
	      which may	also backtrack as necessary; however, should B not
	      match, then no further backtracking will take place, and the
	      pattern will fail	outright at the	current	starting position.

	      The following example counts all the possible matching strings
	      in a pattern (without actually matching any of them).

		  'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
		  print	"Count=$count\n";

	      which produces:

		  aaab
		  aaa
		  aa
		  a
		  aab
		  aa
		  a
		  ab
		  a
		  Count=9

	      If we add	a "(*PRUNE)" before the	count like the following

		  'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
		  print	"Count=$count\n";

	      we prevent backtracking and find the count of the	longest
	      matching string at each matching starting	point like so:

		  aaab
		  aab
		  ab
		  Count=3

	      Any number of "(*PRUNE)" assertions may be used in a pattern.

	      See also "(?>pattern)" and possessive quantifiers	for other ways
	      to control backtracking. In some cases, the use of "(*PRUNE)"
	      can be replaced with a "(?>pattern)" with	no functional
	      difference; however, "(*PRUNE)" can be used to handle cases that
	      cannot be	expressed using	a "(?>pattern)"	alone.

	  "(*SKIP)" "(*SKIP:NAME)"
	      This zero-width pattern is similar to "(*PRUNE)",	except that on
	      failure it also signifies	that whatever text that	was matched
	      leading up to the	"(*SKIP)" pattern being	executed cannot	be
	      part of any match	of this	pattern. This effectively means	that
	      the regex	engine "skips" forward to this position	on failure and
	      tries to match again, (assuming that there is sufficient room to
	      match).

	      The name of the "(*SKIP:NAME)" pattern has special significance.
	      If a "(*MARK:NAME)" was encountered while	matching, then it is
	      that position which is used as the "skip point". If no "(*MARK)"
	      of that name was encountered, then the "(*SKIP)" operator	has no
	      effect. When used	without	a name the "skip point"	is where the
	      match point was when executing the "(*SKIP)" pattern.

	      Compare the following to the examples in "(*PRUNE)"; note	the
	      string is	twice as long:

	       'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
	       print "Count=$count\n";

	      outputs

		  aaab
		  aaab
		  Count=2

	      Once the 'aaab' at the start of the string has matched, and the
	      "(*SKIP)"	executed, the next starting point will be where	the
	      cursor was when the "(*SKIP)" was	executed.

	  "(*MARK:NAME)" "(*:NAME)"
	      This zero-width pattern can be used to mark the point reached in
	      a	string when a certain part of the pattern has been
	      successfully matched. This mark may be given a name. A later
	      "(*SKIP)"	pattern	will then skip forward to that point if
	      backtracked into on failure. Any number of "(*MARK)" patterns
	      are allowed, and the NAME	portion	may be duplicated.

	      In addition to interacting with the "(*SKIP)" pattern,
	      "(*MARK:NAME)" can be used to "label" a pattern branch, so that
	      after matching, the program can determine	which branches of the
	      pattern were involved in the match.

	      When a match is successful, the $REGMARK variable	will be	set to
	      the name of the most recently executed "(*MARK:NAME)" that was
	      involved in the match.

	      This can be used to determine which branch of a pattern was
	      matched without using a separate capture group for each branch,
	      which in turn can	result in a performance	improvement, as	perl
	      cannot optimize "/(?:(x)|(y)|(z))/" as efficiently as something
	      like "/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".

	      When a match has failed, and unless another verb has been
	      involved in failing the match and	has provided its own name to
	      use, the $REGERROR variable will be set to the name of the most
	      recently executed	"(*MARK:NAME)".

	      See "(*SKIP)" for	more details.

	      As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".

	  "(*THEN)" "(*THEN:NAME)"
	      This is similar to the "cut group" operator "::" from Perl 6.
	      Like "(*PRUNE)", this verb always	matches, and when backtracked
	      into on failure, it causes the regex engine to try the next
	      alternation in the innermost enclosing group (capturing or
	      otherwise) that has alternations.	 The two branches of a
	      "(?(condition)yes-pattern|no-pattern)" do	not count as an
	      alternation, as far as "(*THEN)" is concerned.

	      Its name comes from the observation that this operation combined
	      with the alternation operator ("|") can be used to create	what
	      is essentially a pattern-based if/then/else block:

		( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )

	      Note that	if this	operator is used and NOT inside	of an
	      alternation then it acts exactly like the	"(*PRUNE)" operator.

		/ A (*PRUNE) B /

	      is the same as

		/ A (*THEN) B /

	      but

		/ ( A (*THEN) B	| C ) /

	      is not the same as

		/ ( A (*PRUNE) B | C ) /

	      as after matching	the A but failing on the B the "(*THEN)" verb
	      will backtrack and try C;	but the	"(*PRUNE)" verb	will simply
	      fail.

	  "(*COMMIT)" "(*COMMIT:args)"
	      This is the Perl 6 "commit pattern" "<commit>" or	":::". It's a
	      zero-width pattern similar to "(*SKIP)", except that when
	      backtracked into on failure it causes the	match to fail
	      outright.	No further attempts to find a valid match by advancing
	      the start	pointer	will occur again.  For example,

	       'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
	       print "Count=$count\n";

	      outputs

		  aaab
		  Count=1

	      In other words, once the "(*COMMIT)" has been entered, and if
	      the pattern does not match, the regex engine will	not try	any
	      further matching on the rest of the string.

	  "(*FAIL)" "(*F)" "(*FAIL:arg)"
	      This pattern matches nothing and always fails. It	can be used to
	      force the	engine to backtrack. It	is equivalent to "(?!)", but
	      easier to	read. In fact, "(?!)" gets optimised into "(*FAIL)"
	      internally. You can provide an argument so that if the match
	      fails because of this "FAIL" directive the argument can be
	      obtained from $REGERROR.

	      It is probably useful only when combined with "(?{})" or
	      "(??{})".

	  "(*ACCEPT)" "(*ACCEPT:arg)"
	      This pattern matches nothing and causes the end of successful
	      matching at the point at which the "(*ACCEPT)" pattern was
	      encountered, regardless of whether there is actually more	to
	      match in the string. When	inside of a nested pattern, such as
	      recursion, or in a subpattern dynamically	generated via
	      "(??{})",	only the innermost pattern is ended immediately.

	      If the "(*ACCEPT)" is inside of capturing	groups then the	groups
	      are marked as ended at the point at which	the "(*ACCEPT)"	was
	      encountered.  For	instance:

		'AB' =~	/(A (A|B(*ACCEPT)|C) D)(E)/x;

	      will match, and $1 will be "AB" and $2 will be "B", $3 will not
	      be set. If another branch	in the inner parentheses was matched,
	      such as in the string 'ACDE', then the "D" and "E" would have to
	      be matched as well.

	      You can provide an argument, which will be available in the var
	      $REGMARK after the match completes.

   Warning on "\1" Instead of $1
       Some people get too used	to writing things like:

	   $pattern =~ s/(\W)/\\\1/g;

       This is grandfathered (for \1 to	\9) for	the RHS	of a substitute	to
       avoid shocking the sed addicts, but it's	a dirty	habit to get into.
       That's because in PerlThink, the	righthand side of an "s///" is a
       double-quoted string.  "\1" in the usual	double-quoted string means a
       control-A.  The customary Unix meaning of "\1" is kludged in for
       "s///".	However, if you	get into the habit of doing that, you get
       yourself	into trouble if	you then add an	"/e" modifier.

	   s/(\d+)/ \1 + 1 /eg;		   # causes warning under -w

       Or if you try to	do

	   s/(\d+)/\1000/;

       You can't disambiguate that by saying "\{1}000",	whereas	you can	fix it
       with "${1}000".	The operation of interpolation should not be confused
       with the	operation of matching a	backreference.	Certainly they mean
       two different things on the left	side of	the "s///".

   Repeated Patterns Matching a	Zero-length Substring
       WARNING:	Difficult material (and	prose) ahead.  This section needs a
       rewrite.

       Regular expressions provide a terse and powerful	programming language.
       As with most other power	tools, power comes together with the ability
       to wreak	havoc.

       A common	abuse of this power stems from the ability to make infinite
       loops using regular expressions,	with something as innocuous as:

	   'foo' =~ m{ ( o? )* }x;

       The "o?"	matches	at the beginning of ""foo"", and since the position in
       the string is not moved by the match, "o?" would	match again and	again
       because of the "*" quantifier.  Another common way to create a similar
       cycle is	with the looping modifier "/g":

	   @matches = (	'foo' =~ m{ o? }xg );

       or

	   print "match: <$&>\n" while 'foo' =~	m{ o? }xg;

       or the loop implied by "split()".

       However,	long experience	has shown that many programming	tasks may be
       significantly simplified	by using repeated subexpressions that may
       match zero-length substrings.  Here's a simple example being:

	   @chars = split //, $string;		 # // is not magic in split
	   ($whitewashed = $string) =~ s/()/ /g; # parens avoid	magic s// /

       Thus Perl allows	such constructs, by forcefully breaking	the infinite
       loop.  The rules	for this are different for lower-level loops given by
       the greedy quantifiers "*+{}", and for higher-level ones	like the "/g"
       modifier	or "split()" operator.

       The lower-level loops are interrupted (that is, the loop	is broken)
       when Perl detects that a	repeated expression matched a zero-length
       substring.   Thus

	  m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;

       is made equivalent to

	  m{ (?: NON_ZERO_LENGTH )* (?:	ZERO_LENGTH )? }x;

       For example, this program

	  #!perl -l
	  "aaaaab" =~ /
	    (?:
	       a		 # non-zero
	       |		 # or
	      (?{print "hello"}) # print hello whenever	this
				 #    branch is	tried
	      (?=(b))		 # zero-width assertion
	    )*	# any number of	times
	   /x;
	  print	$&;
	  print	$1;

       prints

	  hello
	  aaaaa
	  b

       Notice that "hello" is only printed once, as when Perl sees that	the
       sixth iteration of the outermost	"(?:)*"	matches	a zero-length string,
       it stops	the "*".

       The higher-level	loops preserve an additional state between iterations:
       whether the last	match was zero-length.	To break the loop, the
       following match after a zero-length match is prohibited to have a
       length of zero.	This prohibition interacts with	backtracking (see
       "Backtracking"),	and so the second best match is	chosen if the best
       match is	of zero	length.

       For example:

	   $_ =	'bar';
	   s/\w??/<$&>/g;

       results in "<><b><><a><><r><>".	At each	position of the	string the
       best match given	by non-greedy "??" is the zero-length match, and the
       second best match is what is matched by "\w".  Thus zero-length matches
       alternate with one-character-long matches.

       Similarly, for repeated "m/()/g"	the second-best	match is the match at
       the position one	notch further in the string.

       The additional state of being matched with zero-length is associated
       with the	matched	string,	and is reset by	each assignment	to "pos()".
       Zero-length matches at the end of the previous match are	ignored	during
       "split".

   Combining RE	Pieces
       Each of the elementary pieces of	regular	expressions which were
       described before	(such as "ab" or "\Z") could match at most one
       substring at the	given position of the input string.  However, in a
       typical regular expression these	elementary pieces are combined into
       more complicated	patterns using combining operators "ST", "S|T",	"S*"
       etc.  (in these examples	"S" and	"T" are	regular	subexpressions).

       Such combinations can include alternatives, leading to a	problem	of
       choice: if we match a regular expression	"a|ab" against "abc", will it
       match substring "a" or "ab"?  One way to	describe which substring is
       actually	matched	is the concept of backtracking (see "Backtracking").
       However,	this description is too	low-level and makes you	think in terms
       of a particular implementation.

       Another description starts with notions of "better"/"worse".  All the
       substrings which	may be matched by the given regular expression can be
       sorted from the "best" match to the "worst" match, and it is the	"best"
       match which is chosen.  This substitutes	the question of	"what is
       chosen?"	 by the	question of "which matches are better, and which are
       worse?".

       Again, for elementary pieces there is no	such question, since at	most
       one match at a given position is	possible.  This	section	describes the
       notion of better/worse for combining operators.	In the description
       below "S" and "T" are regular subexpressions.

       "ST"
	   Consider two	possible matches, "AB" and "A'B'", "A" and "A'"	are
	   substrings which can	be matched by "S", "B" and "B'"	are substrings
	   which can be	matched	by "T".

	   If "A" is a better match for	"S" than "A'", "AB" is a better	match
	   than	"A'B'".

	   If "A" and "A'" coincide: "AB" is a better match than "AB'" if "B"
	   is a	better match for "T" than "B'".

       "S|T"
	   When	"S" can	match, it is a better match than when only "T" can
	   match.

	   Ordering of two matches for "S" is the same as for "S".  Similar
	   for two matches for "T".

       "S{REPEAT_COUNT}"
	   Matches as "SSS...S"	(repeated as many times	as necessary).

       "S{min,max}"
	   Matches as "S{max}|S{max-1}|...|S{min+1}|S{min}".

       "S{min,max}?"
	   Matches as "S{min}|S{min+1}|...|S{max-1}|S{max}".

       "S?", "S*", "S+"
	   Same	as "S{0,1}", "S{0,BIG_NUMBER}",	"S{1,BIG_NUMBER}"
	   respectively.

       "S??", "S*?", "S+?"
	   Same	as "S{0,1}?", "S{0,BIG_NUMBER}?", "S{1,BIG_NUMBER}?"
	   respectively.

       "(?>S)"
	   Matches the best match for "S" and only that.

       "(?=S)",	"(?<=S)"
	   Only	the best match for "S" is considered.  (This is	important only
	   if "S" has capturing	parentheses, and backreferences	are used
	   somewhere else in the whole regular expression.)

       "(?!S)",	"(?<!S)"
	   For this grouping operator there is no need to describe the
	   ordering, since only	whether	or not "S" can match is	important.

       "(??{ EXPR })", "(?PARNO)"
	   The ordering	is the same as for the regular expression which	is the
	   result of EXPR, or the pattern contained by capture group PARNO.

       "(?(condition)yes-pattern|no-pattern)"
	   Recall that which of	"yes-pattern" or "no-pattern" actually matches
	   is already determined.  The ordering	of the matches is the same as
	   for the chosen subexpression.

       The above recipes describe the ordering of matches at a given position.
       One more	rule is	needed to understand how a match is determined for the
       whole regular expression: a match at an earlier position	is always
       better than a match at a	later position.

   Creating Custom RE Engines
       As of Perl 5.10.0, one can create custom	regular	expression engines.
       This is not for the faint of heart, as they have	to plug	in at the C
       level.  See perlreapi for more details.

       As an alternative, overloaded constants (see overload) provide a	simple
       way to extend the functionality of the RE engine, by substituting one
       pattern for another.

       Suppose that we want to enable a	new RE escape-sequence "\Y|" which
       matches at a boundary between whitespace	characters and non-whitespace
       characters.  Note that "(?=\S)(?<!\S)|(?!\S)(?<=\S)" matches exactly at
       these positions,	so we want to have each	"\Y|" in the place of the more
       complicated version.  We	can create a module "customre" to do this:

	   package customre;
	   use overload;

	   sub import {
	     shift;
	     die "No argument to customre::import allowed" if @_;
	     overload::constant	'qr' =>	\&convert;
	   }

	   sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"}

	   # We	must also take care of not escaping the	legitimate \\Y|
	   # sequence, hence the presence of '\\' in the conversion rules.
	   my %rules = ( '\\' => '\\\\',
			 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ );
	   sub convert {
	     my	$re = shift;
	     $re =~ s{
		       \\ ( \\ | Y . )
		     }
		     { $rules{$1} or invalid($re,$1) }sgex;
	     return $re;
	   }

       Now "use	customre" enables the new escape in constant regular
       expressions, i.e., those	without	any runtime variable interpolations.
       As documented in	overload, this conversion will work only over literal
       parts of	regular	expressions.  For "\Y|$re\Y|" the variable part	of
       this regular expression needs to	be converted explicitly	(but only if
       the special meaning of "\Y|" should be enabled inside $re):

	   use customre;
	   $re = <>;
	   chomp $re;
	   $re = customre::convert $re;
	   /\Y|$re\Y|/;

   Embedded Code Execution Frequency
       The exact rules for how often "(??{})" and "(?{})" are executed in a
       pattern are unspecified.	 In the	case of	a successful match you can
       assume that they	DWIM and will be executed in left to right order the
       appropriate number of times in the accepting path of the	pattern	as
       would any other meta-pattern.  How non-accepting	pathways and match
       failures	affect the number of times a pattern is	executed is
       specifically unspecified	and may	vary depending on what optimizations
       can be applied to the pattern and is likely to change from version to
       version.

       For instance in

	 "aaabcdeeeee"=~/a(?{print "a"})b(?{print "b"})cde/;

       the exact number	of times "a" or	"b" are	printed	out is unspecified for
       failure,	but you	may assume they	will be	printed	at least once during a
       successful match, additionally you may assume that if "b" is printed,
       it will be preceded by at least one "a".

       In the case of branching	constructs like	the following:

	 /a(b|(?{ print	"a" }))c(?{ print "c" })/;

       you can assume that the input "ac" will output "ac", and	that "abc"
       will output only	"c".

       When embedded code is quantified, successful matches will call the code
       once for	each matched iteration of the quantifier.  For example:

	 "good"	=~ /g(?:o(?{print "o"}))*d/;

       will output "o" twice.

   PCRE/Python Support
       As of Perl 5.10.0, Perl supports	several	Python/PCRE-specific
       extensions to the regex syntax. While Perl programmers are encouraged
       to use the Perl-specific	syntax,	the following are also accepted:

       "(?P<NAME>pattern)"
	   Define a named capture group. Equivalent to "(?<NAME>pattern)".

       "(?P=NAME)"
	   Backreference to a named capture group. Equivalent to "\g{NAME}".

       "(?P>NAME)"
	   Subroutine call to a	named capture group. Equivalent	to "(?&NAME)".

BUGS
       There are a number of issues with regard	to case-insensitive matching
       in Unicode rules.  See "i" under	"Modifiers" above.

       This document varies from difficult to understand to completely and
       utterly opaque.	The wandering prose riddled with jargon	is hard	to
       fathom in several places.

       This document needs a rewrite that separates the	tutorial content from
       the reference content.

SEE ALSO
       The syntax of patterns used in Perl pattern matching evolved from those
       supplied	in the Bell Labs Research Unix 8th Edition (Version 8) regex
       routines.  (The code is actually	derived	(distantly) from Henry
       Spencer's freely	redistributable	reimplementation of those V8
       routines.)

       perlrequick.

       perlretut.

       "Regexp Quote-Like Operators" in	perlop.

       "Gory details of	parsing	quoted constructs" in perlop.

       perlfaq6.

       "pos" in	perlfunc.

       perllocale.

       perlebcdic.

       Mastering Regular Expressions by	Jeffrey	Friedl,	published by O'Reilly
       and Associates.

perl v5.26.0			  2017-04-19			     PERLRE(1)

NAME | DESCRIPTION | BUGS | SEE ALSO

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